The relationship of conventional semen parameters and the limits of these parameters for fertilization in vitro were analyzed from data over a 3-year period (1980 to 1982). Sperm motility was the single most important parameter determining the fertilization rate. Fertilization failed when the initial and final motilities were less than 20% and 30%, respectively. The percentage of abnormal sperm forms was also significantly related to the fertilization rate; but even when there were greater than 60% abnormal spermatozoa, fertilization could be obtained. Sperm concentration in semen had no significant effect on the fertilization rate when the data were controlled for motility or abnormal sperm forms. The fertilization rate increased with reduced sperm numbers used for insemination in vitro but had no effect on the incidence of multiple pronuclei in oocytes.