The crude antigenic fraction (CAF) isolated from Leishmania major was fractionated into three subfractions by sodium dodecyl sulphate-polyacrylamide gel (SDS-PAGE). The effects of CAF and its subfractions on NK cell cytotoxicity is investigated by chromium release assay. These subfractions designated as fractions 1, 2 and 3 correspond to 97.4-66 kD, 66-45 kD and 29 kD and below respectively. Although both CAF and its subfractions have inhibited the cytotoxicity of natural killer (NK) cells, the effects of fractions 2 and 3 were more pronounced. The effect of the fractions on the Interferon-gamma (IFN-gamma) and Interleukin-4 (IL-4) secretion by peripheric blood lymphocytes was also analyzed. It was found that CAF and fraction 1 induce IFN-gamma secretion while on the other hand IL-4 secretion was mostly suppressed by fraction 2. Therefore, further research is being executed which focuses on the effects of CAF, fractions 1 and 2 on macrophage effector functions. (C) 1997 Elsevier Science B.V.