Experiments were conducted to evaluate the effects of supernatants from activated leukocyte cultures and purified lymphokine and monokine preparations on sperm motion. An automated semen analyzer was used to obtain accurate measurements of several parameters of sperm motion. Supernatants from activated peripheral blood leukocyte cultures significantly decreased sperm motility. In studies of effects of individual lymphokines and monokines, significant antimotility effects were observed when spermatozoa were incubated with the lymphokine gamma-interferon and the monokine tumor necrosis factor. Subnormal fertility may result from defective sperm function caused by lymphokines and monokines elaborated by activated lymphocytes and macrophages residing in the reproductive tracts of infertile men and women.