Human semen specimens with low motility were incubated with L-arginine to determine its effect on sperm motility. Stimulation of forward motility over control values occurred in a dose response relationship, with optimal activity occurring at an L-arginine concentration of 0.004 M. L-arginine increased the forward motility of sperm at either 23 or 37°C in all samples tested. The increase in forward motility over control values was 81.6 ± 9.96 percent SEM (n=12). The percent stimulation of sperm motility was in general inversely related to the initial semen motility. The physiologically inactive isomer D-arginine, the amino acid L-lysine and the arginine analogs L-homoarginine and L-nitroarginine did not stimulate sperm motility. L-ornithine, an arginine metabolite, was also ineffective in stimulating sperm motility. The observation that L-arginine enhances sperm motility in vitro suggests that it may have clinical value when human semen with subnormal motility is utilized for artificial insemination.