Summary. When ejaculated bovine semen was washed twice with Ficoll the spermatozoa, resuspended in buffer, became immotile. Motility could be restored by addition of seminal plasma from vasectomized bulls and by addition of both bovine serum albumin (BSA) and theophylline. Motility could be restored and maintained at 37°C to a variable extent with BSA alone. When this motility ceased it could be revived with theophylline or seminal plasma. When spermatozoa, inactivated by washing with Ficoll, were reactivated with seminal plasma the time that motility lasted at 37°C before it was irreversibly lost depended on the concentration of seminal plasma: the more seminal plasma added, the shorter the duration of motility. This suggested that seminal plasma contained separate factors which restored motility and led to permanent inactivation of the spermatozoa. A motility-stimulating factor was present in the low molecular weight fraction of seminal plasma which had been passed through an ultrafilter of retention Mr 500 and the damaging effect was confined to the high molecular weight non-dialysable fraction.