Effects of sperm immobilizing antibodies on sperm penetration through human zonae pellucidae have been studied. Exposure of human spermatozoa obtained from fertile donors to seven serum samples with sperm immobilizing antibody impaired sperm penetration completely in six cases and incompletely in one case. During the course of treatment of a patient with circulating sperm immobilizing antibody by means of an in vitro fertilization and embryo transfer program, it was found that fertilization was completely blocked in the presence of the patient's serum, but three matured ova fertilized successfully when umbilical cord serum was used instead of autoserum from the patient. Furthermore, when spermatozoa were exposed to an IgG fraction of sera containing sperm immobilizing antibody, sperm binding and penetration were markedly inhibited. The spermatozoa, preincubated with sperm immobilizing antibody, showed penetrability across the zona pellucida. However, exposure of possibly capacitated sperm to the antibody completely blocked sperm binding to and penetration through the zona pellucida. These results suggest that sperm immobilizing antibodies cause infertility by preventina sperm binding to and penetration through the zona pellucida, possibly by interfering with the step of fertilization beyond sperm capacitation.