Naturally occurring human antisperm antibodies (ASA) detectable by immunofluorescence, were found to have a peak indicence of 90% in both sexes before puberty. Thereafter, the incidence declined to about 60% and persisted through life. This age-related incidence resembled that for foreign and not self antigens. The natural incidence of the six antibodies under investigation varied: the most frequent were antibodies to acrosomal antigen Acl and equatorial antigen; the least frequent were antibodies to acrosomal antigen Ac2 and sperm nuclear protamine; and between these were antibodies to the mainpiece of tail and post-acrosomal region. Irrespective of their natural incidence, these antibodies increased at a comparable rate in men following vasectomy indicating these six sperm antigens have comparable immunogenicity. These results provide evidence for the lack of immunologic tolerance in man toward many sperm antigens. The variation in the natural incidence of individual ASA is explanable by differences in prevalence of crossreaction between each sperm antigen with exogenous antigens. Thus, antibodies to acrosomal antigen Acl and equatorial antigen, which occurred most frequently, appeared to crossreact with ubiquitous microorganisms.