The migration of sperm (expressed as the percentage of the total motile cells in the ejaculate aliquot) from seminal plasma into overlying medium (swim-up) was increased with time, temperature, and increasing the surface area of contact between the two phases by increasing the angle of tube inclination.The proportion of sperm migrating into the medium was relatively constant with varying sperm count. Stimulation of the progressive motility of spermatozoa in vitro using a motility enhancing drug (2-deoxyadenosine) increased sperm migration initially but no more than the maximum reached by controls after extended incubation.Sperm yield from normozoospermic men and men with borderline asthenoteratozoospermia represented approximately 40 and 15% of the total motile cells in the ejaculate aliquots, respectively. In general, in both groups spermatozoa in the swim-up fraction exhibited superior motility, vitality, and morphology compared to the original ejaculates. Tail abnormalities were reduced more than head abnormalities and subjects with borderline sperm qualities exhibited less dramatic improvements in quality than the normozoospermic individuals.