We fractionated the whole human seminal plasma on DEAE Sephadex A-50 ion exchange columns. Complete separation was achieved in seven peaks using different salt concentrations in phosphate buffer pH 6. The seminal plasma proteins were separated by sodium dodecyl-sulphate polyacrylamide gel electrophoresis. Human seminal plasma (SP) and its fractions were used in mixed lymphocyte reaction in vitro. Fractions 3, 4, and 7 were found to suppress the proliferation of human peripheral blood mononuclear cells to phytohemagglutinin and pokweed mitogen at a concentration of 10 micrograms ml-1 while stimulatory effect was observed at lower concentrations (1 microgram and 2.5 micrograms ml-1). Whole human SP and other fractions failed to suppress the proliferation of lymphocytes in vitro. Furthermore, the effect of human SP and its fractions was also investigated on phagocytic function of polymorphonuclear leukocytes (PMNs) using luminol dependent chemiluminescence assay stimulated with phorbol myristate acetate and opsonized yeast. Fractionated SP was found to have a suppressive effect on the luminol-dependent chemiluminescence of PMNs in the whole blood.