A prospective study was carried out on semen samples from 118 consecutive unselected men attending our infertility clinic to determine whether sperm motility may be affected by seminal plasma. The incidence of asthenozoospermia as defined by fewer than 50% of spermatozoa with forward progressive motility in the untreated semen was 37.4%. This value was significantly reduced to 23% after washing and removing seminal plasma. Men with asthenozoospermia in untreated semen but normal in the washed sample had a percentage of normal sperm morphology and a percentage of swollen tails in the HOS test similar to those of controls, and higher than those of asthenozoospermics in both the untreated and washed sample. Sperm velocity was also significantly improved after the washing procedure. Spermatozoa selected by swim-down procedure and applied to a seminal plasma medium reduced sperm motility affecting negatively the HOS test. Sperm motility should be assessed after a sperm washing procedure, since seminal plasma contains constituents that decrease sperm motility without affecting membrane integrity.