Optimization of each step in the standard protocol of the sperm penetration assay has increased the sensitivity of the traditional assay, and this increased sensitivity has allowed us to avoid the production of false-positive results. Development and use of routine quality control methods now enable us to compare different samples from a given individual and monitor with confidence biological variation and response to medical or surgical treatment. Although our test is extremely predictive of how well a patient's specimen may do as a result of normal intercourse, artificial insemination, and human in-vitro fertilization, it must be stressed that the sperm penetration assay and the sperm capacitation index reported by our laboratory only measure the ability of a sperm to capacitate and fuse with zona-free hamster ovum. How well spermatozoa disperse the cumulus and bind to and penetrate the human zona pellucida is not measured by this test. This would be reflected by an apparent "false-positive" outcome in the sperm penetration assay. However, our data from 138 patients suggest that this occurs only on rare occasions (in 2 of the 138). The test can be used only as a supplement to a conventional semen analysis because other factors, such as motile concentration and motility characteristics, clearly are also of importance particularly in vivo. The results obtained by our laboratory using the optimized assay described herein demonstrate that this test can play an important role in the evaluation of an infertile man.