The present study was designed to test the validity of the hamster egg penetration assay as a prognostic indicator of male fertility in 54 patients undergoing in vitro fertilization. Human oocyte fertilization, cleavage, and pregnancy were compared with the results of this bioassay. Good correlation was found between hamster egg penetration and oocyte fertilization. Conversely, a definite lower limit of hamster egg penetration to define absolute male infertility could not be established because human oocyte fertilization, cleavage, and even pregnancy occurred in spite of low hamster egg penetration.