The dialysable heat-stable fraction of human serum stimulates the motility of hamster epididymal spermatozoa in vitro (Yanagimachi, 1970) and produces favourable conditions for capacitation, although the presence of protein is also required to stimulate the acrosome reaction efficiently (Yanagimachi, 1970; . The active fraction from gel-filtered human serum was able to substitute for the motility-stimulating activity normally associated with the hamster cumulus mass (Bavister & Morton, 1974),

more »

... d it is reasonable to assume at present that the active substances from both sources are similar. Some properties of the serum-derived substance that may assist in its identification are described in the present communication.