Donor spermatozoa with good motility were pretreated with four sera containing high titers of sperm-agglutinating IgG, one serum without sperm-agglutinating activity, the IgG fractions from these five sera, F(ab)2 and Fragment antigen binding (Fab) fragments from these sera and from the IgG fractions, and one seminal plasma sample with a high titer of sperm-agglutinating IgA. With mixed antiglobulin reaction tests the percentage of motile pretreated spermatozoa sensitized with IgG Fab or IgG Fragment crystalline (Fc) parts was determined. Spermatozoa sensitized with intact antispermatozoal IgG showed a strong reduction in their capacity to penetrate cervical mucus. The reduction of the penetration capacity was determined by estimation of the percentage of motile spermatozoa rapidly shaking (S%) in the sperm-cervical mucus contact (SCMC) test. Removal of the Fc parts resulted in a decreased S%. Treatment of spermatozoa, on which Fab fragments were present, with intact antibodies to IgG Fab fragments, resulted in a recurrence of a high S%. A decrease of the S% was also found if Fab fragments from antibodies to IgG Fc fragments were added to spermatozoa sensitized with intact antispermatozoal IgG. Similarly, it was found that a decrease of the S% occurred when IgA sensitized spermatozoa were treated with Fab fragments from antibodies to human IgA. In the sperm penetration meter test the IgA sensitized spermatozoa treated with Fab fragments from anti-human IgA antibodies showed a better penetration than untreated IgA-sensitized spermatozoa.