Notwithstanding direct detection of microbial/viral pathogens or their associated toxins, the quality of drinking water may also be evaluated according to its pro-inflammatory potential. In this latter setting, contamination with pathogens or their products is determined according to the magnitude of activation of blood-derived immune/inflammatory cells following exposure to test water samples in vitro, usually by monitoring the synthesis of pro-inflammatory cytokines. The primary objective of the current study was to apply this procedure to evaluate the pro-inflammatory potential of water sampled at entry, as well as at various stages of treatment, from 3 major water treatment facilities in the greater Pretoria region, viz., the Daspoort, Hartbeespoort, and Rietvlei Water Treatment Facilities. Control water samples included domestic tap water, bottled water from a commercial source, and distilled water. Peripheral blood mononuclear leukocytes (MNL) were isolated from the blood of healthy, adult, human volunteers (n=3), enumerated, suspended in tissue culture medium RPMI 1640 containing antibiotics at a concentration of 1x106/mℓ, and exposed to the various water samples (10%) for 18 h at 37°C. Following incubation, the cell-free supernatants were assayed for the cytokine, interleukin-6 (IL-6), using a quantitative, sandwich, enzyme immunoassay procedure. The mean values for the untreated control system and for a positive control system exposed to bacterial endotoxin (120 ng/mℓ) were 153.5 ±17 and 1 561 ±30 pg/mℓ, respectively (p= 0.03). The production of IL-6 was unaffected following exposure of MNL to the control water samples. However, inlet water sampled from all three facilities, especially Hartbeespoort, resulted in significant activation of production of IL-6 by MNL, which declined with progressive treatment, consistent with removal of pro-inflammatory contaminants. Surprisingly, however, a rebound in pro-inflammatory activity was evident in outlet water sampled from Hartbeespoort. In conclusion, the results of the current study appear to support the efficiency of water treatment procedures at the Daspoort and Rietvlei Treatment Facilities, while confirming the usefulness of IL-6-based assays as adjuncts to conventional water quality testing procedures.