The presence of IgG and IgA on motile spermatozoa from normal semen donors and men from infertile couples was studied with mixed antiglobulin reaction (MAR) tests.The percentage of motile spermatozoa with IgG (IgG MAR%) was found to be related to the circulating antispermatozoal IgG. No direct relation could be detected between the IgG MAR% and the sperm agglutinating activity in seminal plasma (SP) or the percentage of motile spermatozoa showing the shaking phenomenon (S%) in the sperm cervical mucus contact (SCMC) test. The percentage of motile spermatozoa with IgA (IgA MAR%) showed no direct relation to the sperm agglutinating activity in serum and SP, but was roughly proportional with the S%.It was discerned that the shaking phenomenon in the SCMC test was probably due to presence of IgA on the motile spermatozoa. Previously it had been demonstrated that also the sperm agglutinating activity in SP is caused by IgA that is probably locally produced in the male genital tract. In conclusion it was thought that the reduced ability of penetration into cervical mucus by spermatozoa from infertile men is caused by locally produced IgA.