The motility of ejaculated human sperm which was washed thoroughly by Ficoll gradient centrifugation to remove seminal plasma was investigated in the presence of various concentrations of sugars. Washed human sperm showed high motility in an artificial medium containing glucose and fructose, which were major sugar components of semen. However, mannitol and 3-O-methyl-D-glucose had no effect on sperm motility. Dose-response curves of sperm motility in the additions of glucose or fructose were bell-shaped. The maximum sperm motility was found in the medium with 1 mM glucose or 15 mM fructose. When, however, both 1 mM glucose and 15 mM fructose were added to the medium, sperm motility was higher than that with an individual sugar. Although phlorizin decreased glucose activated sperm motility, it has no effect in case fructose was the activator. The results suggest that the effect of glucose on human sperm motility is due to a different mechanism from that of fructose.