A number of studies have implied that seminal plasma contains one or more factors that stimulate spermatozoal motility. On the other hand, many authors have shown that continuous, long term exposure to seminal plasma is detrimental to motility and survival of spermatozoa. In this study we examined the long term effects of human seminal plasma not only from whole semen but also from different fractions of split ejaculate on motility of human ejaculated spermatozoa. Washed spermatozoa were incubated in modified BWW medium containing human serum albumin with or without seminal plasma. A considerable percentage of spermatozoa maintained good progressive motility for up to 20 h. when free from seminal plasma. Addition of seminal plasma (20%, v/v) obviously depressed the motility, particularly progressive motility. The most depressive effect occurred in the first (prostatic) fraction of split ejaculate, and the effect of the last (vesicular) fraction was less harmful. These observations suggest that seminal plasma has (a) factor(s) that impair the maintenance of spermatozoal motility which originate possibly from the prostate or epididymis, since both fluids of these accessory organs appear to be secreted mainly into the first portion ejaculate.