Aprotinin and a seminal plasma factor provide two new tools to study the regulation of sperm motility.

A seminal plasma factor and aprotinin are capable of inhibiting the movement of demembranated reactivated rabbit spermatozoa. These inhibitors can be overcome by the addition of ATP. This reversibility could be explained by a common binding receptor for the inhibitors and MgATP. The respective affinities for the receptor are dependent on whether the microtubules are sliding or resting.