In an examination of 34 southern African SAT-type foot-and-mouth disease viruses, all but 1 attained satisfactory levels of infectivity within 6 passages in rolled BHK₂₁monolayer cell cultures. However, there were marked differences between adapted viruses with respect to the mass of immunogen (146S material) produced. Several isolates which consistently produced levels ≥2 µg/ml were identified. In cross neutralization tests using post-vaccinal sera, SAT-1 and SAT-2 isolates showed considerable diversity and none of the viruses tested would be expected to produce a broad-spectrum response if incorporated into a vaccine. On the other hand, when 2 of the SAT-2 isolates were incorporated into the same vaccine a distinctly broader response resulted.