Male partners of 129 infertile couples were evaluated for antisperm factors by using the hamster zona-free ovum (HZFO) test. Sperm-agglutinating activity and antifertilization factors were assessed by adding the male partner's serum to donor sperm in the HZFO-serum antibody (HA) test. When patient's HZFO scores were less than 20% of the hamster ova fertilized, fertilization by patient's serum in the HA test. Of 13 samples that failed to fertilize any of the hamster ova, nine were antibody-negative, of which seven had at least one extremely deficient sperm parameter. Of the four men with positive serum, two had normal and one had abnormal semen (one did not have a complete semen analysis). These data emphasize the need to test male partners for a decline in fertilizing capacity, which is frequently related to the presence of serum antisperm factors. These tests could be used to identify paternal and maternal components (sperm and serum) that may hinder fertilization in fertilization programs.