ABSTRACT: Several reports have demonstrated the presence of anti-sperm antibodies (ASA) in infertile populations; however there is a paucity of information regarding ASA in fertile populations. The purpose of this study was to establish objective criteria for the interpretation of the Immuno-bead Binding Test (IBT) based on values obtained from fertile individuals. Sera from 20 fertile couples (n = 40) were assayed by using a modification of the IBT previously described by Clarke et al. An initial lower limit of binding for positivity (lower limit) of 14% was used based upon the mean value for each isotype plus 2 standard deviations (SD) of 4 negative control sera assayed 4 to 7 times each. One-way ANOVA or chi-square analyses were used to analyze these data. There was no difference in percent immunobead binding between males and females in the fertile population (P > 0.1); therefore the data were pooled. Percent binding for fertile controls was: IgG, 21.7 + 31.9% (mean + SD); IgA, 19.5 + 25.8%; IgM, 16.9 + 14.9%. Initial analysis indicated no significant difference (P > 0.10) in percent binding between fertile and infertile individuals. The corresponding frequency of positive values (for at least one isotype) using a 14% lower limit was 23/40 (57.5%). This was not significantly different (P > 0.1) from the frequency observed in the patient population (140/ 242, 57.8%). New lower limits of positivity for each isotype were established based upon the mean plus 2 SD from the fertile control data: IgG, 85.4%; IgA, 71.1%; IgM, 46.7%. The frequencies of positive values obtained by using these criteria were not different (P > 0.1) between fertile and patient populations (20.0 vs. 18.7%, respectively). In addition, neither the number of beads bound per sperm nor the location of bead binding differed (P > 0.10) between the 2 populations. These data indicate that the frequency of circulating ASA is similar in fertile and infertile populations and suggests that ASA testing may be of questionable clinical significance.