To investigate the role of seminal plasma in human B cell functions, its effect on the proliferation and antibody secretion of tonsillar B cells and an Epstein-Barr virus (EBV) transformed human B cell line, A4, was examined. Seminal plasma inhibited both the proliferation and differentiation of normal B cells only when added to the cultures at the early period of culture. If addition of seminal plasma was delayed beyond 5 to 6 days, it failed to inhibit IgG secretion. Seminal plasma did not show any inhibitory effect on A4 cells, but rather enhanced both the proliferation and IgG secretion of this B cell line. When the low and high mol. wt fractions of seminal plasma were tested for their biological effects on normal and transformed B cells, the low mol. wt fraction (less than 1 kD) was associated with the inhibitory effect of seminal plasma on normal B cells, whereas high mol. wt fractions (both dialysed and 1500-kD fraction) was involved in the enhancing effect on A4 cells. We conclude that (i) seminal plasma inhibits the early proliferation of normal human B cells, but does not inhibit the antibody-secreting capacity of mature B cells; and (ii) different molecules of seminal plasma act on the different stages of B cell maturation.