This study compared the effects of human follicular fluid (hFF) from women with endometriosis, tubal factor and male factor on the zona binding capacity of human spermatozoa. Samples of hFF were collected from 30 patients, 10 patients for each of the indications of infertility, at the time of oocyte retrieval in an in-vitro fertilization/embryo transfer programme. The hemizona binding assay (HZA) was used to assess the effect of these hFF on the zona binding potential of human spermatozoa. The mean numbers of spermatozoa bound to the zona pellucida after treating the spermatozoa with hFF from endometriosis, tubal factor and male factor were 90.5 +/- 20.9, 108.9 +/- 22.3 and 101.2 +/- 13.4 respectively. These were significantly lower than their corresponding controls, the spermatozoa of which were incubated with Earle's balanced salt solution (endometriosis 238.7 +/- 34.7; tubal factor 210.8 +/- 41.6; male factor 205.4 +/- 26.3; P <0.002). The hemizona binding index (HZI) was similar between male factor samples (52.0 +/- 6.7) and tubal factor samples (53.8 +/- 4.2). Spermatozoa incubated with hFF from endometriosis patients (36.0 +/- 4.1) had an HZI that was significantly lower than those treated with hFF from tubal factor patients (P <0.01). Probably due to small sample size, the differences in HZI between endometriosis samples and male factor samples did not reach statistical significance (P = 0.076). These data suggest that there was a stronger sperm-zona binding inhibitory effect of hFF from patients with endometriosis than from those without the disease.