The effects of incubation of spermatozoa with three serotypes of Ureaplasma urealyticum on spermatozoal motility and penetration in vitro were investigated. Using computer-assisted videomicroscopy, three parameters of motility were determined: individual path lengths, individual vectorial distances, and percentage motility. Polyacrylamide gels were used as a medium for assessment of spermatozoal penetration. Ureaplasma-infected spermatozoa did have significantly greater path lengths and individual distances than did uninfected controls, but ureaplasma infection had no significant effect on percentage motility. Overall, there were no significant differences in penetration distances between ureaplasma-infected spermatozoa and their corresponding uninfected controls. Our conclusion is that the ureaplasmas did not adversely affect motility or penetration when spermatozoa were incubated with ureaplasmas for 45 minutes at ureaplasma:sperm ratios as high as 100:1.