The one-way mixed lymphocyte culture (MLC) is used to assess histocompatibility between donor and recipient. First introduced in 1966, this method involves the co-culture of lymphocytes from the peripheral blood of the donor and the recipient for a period of 6 to 7 days: antigen disparities, primarily in the HLA-DR region, stimulate proliferation of the responding cells, which is detected by addition of 3H-labelled thymidine and subsequent measurement of radioactivity. The lymphocytes of either the donor, used to predict graft-versus-host disease (GVHD) or recipient, used to predict host-versus-graft disease (HVGD)/graft rejection, are inactivated by exposure to radiation or mitomycin C, so that the observed proliferation is that of the other set of lymphocytes, hence the name “one-way” MLC. The amount of measured radioactivity is directly proportional to the amount of DNA synthesized, which is a reflection of the number of disparities at the major histocompatibility complex (MHC).Previous studies have established that inactivation of the lymphocytes by radiation and mitomycin C, has a negative effect on the structure/expression of HLA-DR molecules on the cell surface, which provides the primary stimulus for the MLC reaction. The laboratory research presented in this dissertation was designed i) to compare the viabilities and HLA-DR levels on stimulator cells exposed to cyclosporin A, mitomycin C and ionizing irradiation , in order to determine whether cyclosporin A can be used as an alternative to mitomycin C or radiation as inactivator of the stimulator cells in the one-way MLC; ii) to improve sensitivity and accelerate the MLC reaction by addition of IL-2; iii) establish a flow cytometric mixed lymphocyte assay using the fluorochrome 5,6 carboxyfluorescein diacetate succinimidyl ester (CFSE). Cyclosporin A showed striking similarities to mitomycin C and ionizing radiation in its effect on viability and reduction/structural changes in HLA-DR molecules of the stimulator cells. Exposure of the stimulator cells to 20ìM cyclosporin A, demonstrated a significant loss of both cell viability and HLA-DR molecule cell surface expression. Thus, in evaluating these three methods of inactivation of the stimulator cells in the one-way MLC, it was concluded that a one-way MLC may not in fact be an accurate and qualitative reflection of the histocompatibility between donor and recipient. Instead the two-way MLC , in which neither the donor’s nor recipient’s cells are inactivated, may be a more reliable alternative. The only limitation associated with a two-way MLC is the inability to distinguish between a host-versus-graft-rejection and a graft-versus-host reaction in the observed allogeneic response Addition of 5 and 10 IU/ml IL-2 to the MLC showed the opposite effect to that intended, inhibiting proliferation in the MLC. Previous studies have shown that an excess of IL-2 results in the production of suppressor T cells. The amount of IL-2 produced during the MLC depends on the number of disparities in the MLC between donor and recipient, which will be different for each MLC reaction. Since the number of allogeneic T cells involved in the MLC reaction is not known, the amount of IL-2 produced during the allogeneic immune response in the MLC can not be predicted and addition of exogenous IL-2 may result in production of suppressor T cells and an inhibition of proliferation. The two-way MLC was modified by staining one of the participating set of lymphocytes (donors or recipients) with CFSE and tracking proliferation in this population, using flow cytometry. The two-way CFSE-based MLC analyzed in this study were counterstained with CD25 (IL-2R). An increase in CD25 expression on the cell surface is an indicator of cell activation and proliferation. Proliferation, as indicated by a progressive loss of CFSE fluorescence correlated well with the corresponding increase in CD25 expression and accumulated daughter cells. In addition, by loading only one of the participating donors in the two-way MLC, the responder/stimulator interaction, observed in the one-way MLC, is re-established. Thus the modified, CFSE-based two-way MLC can be used to predict GVHD. To conclude, the use of CFSE labeling and flow-cytometry to measure proliferation in a two-way MLC, together with CD25 counterstaining provides an alternative, reliable and probably superior method to 3H thymidine uptake.