From 1967 to 1973 serum samples of 1709 infertile women were tested for antispermatozoal antibodies with the gelatin agglutination test. In 110 cases sperm agglutinins were demonstrated in titers ranging from 4 to 1024. The clinical data of 99 couples were evaluable, and in 65 there appeared to be unexplained infertility. Out of these 65 women, 44 became pregnant in the posttesting follow-up period of 6 to 13.5 years (median, 10 years). With increasing titers there was a significant decrease in the probability of becoming pregnant. Of the women who became pregnant, in general, those with higher titers had to wait longer for their pregnancy than those with lower titers. Moreover, with increasing titers there was a tendency toward a longer duration of infertility before the serum was tested. This latter observation supports the contention that the agglutinin titer influences the fecundability rate. The titers of those becoming pregnant and those remaining infertile, however, showed considerable overlap. Favorable results in the postcoital test were associated with a better prognosis, but this was independent of the titers.