The effect on sperm motility of sperm-sperm and sperm-seminal plasma interactions was studied among homologous and heterologous sperm. There were no significant interactions between sperm in vitro, but it was found that seminal plasmas of different donors have different effects on sperm motility, and different sperm react differently to the same seminal plasma. Sperm showed higher motility in a pure physiological solution than when mixed with seminal plasma, even if the plasma and sperm came from the same donor. Various plasma components are responsible for this modulation of sperm motility. It would appear that large numbers of sperm are adaptive, among other things, because they are involved in sperm selection.