Wistar rats were inoculated with purified YWK-I antibody. The anti-idiotypic antibodies were isolated from rat sera by successive passage over affinity chromatography columns of YWK-I mAb and normal mouse Igs. Specificity of anti-Id antibody was established by ELISA. The 84 kD protein inhibited the binding of anti-Id to YWK-I mAb, but failed to repress antibody against normal mouse Ig binding to YWK-I mAb. In eompetitive inhibition assay, 84kD protein had shown the ability 4o compete with anti-Id binding to YWK-I mAb in a dose-dependent manner. Crude sperm extract showed a lower competitive ability. No effect was found with the irrelevant 36 kD sperm protein. The antisera from the Balb/c mice immunized with AId contained Ab3 that reacted with 84 kD sperm protein. The binding of anti-Id to YWK-I mAb was inhibited by Ab3 in a dose-dependent fashion and Ab3 was shown to be able to induce human sperm agglutination. These results indicate that anti-Id which may mimic an epitope of the 84 kD protein could be exploited as an antigen to raise antibodies against sperm protein. Wistar rats were inoculated with purified YWK-I antibody. The anti-idiotypic antibodies were isolated from rat sera by following passage over affinity chromatography columns of YWK-I mAb and normal mouse Igs. Specificity of anti-Id antibody was established by ELISA. The 84 kD protein inhibited the binding of anti-Id to YWK-I mAb, but failed to repress antibody against normal mouse Ig binding to YWK-I mAb. In eompetitive inhibition assay, 84 kD protein had shown the ability 4 o compete with anti-Id binding No effect was found with the irrelevant 36 kD sperm protein. The antisera from the Balb / c mice immunized with AId contained Ab3 reacted with 84 The binding of anti-Id to YWK-I mAb was inhibited by Ab3 in a dose-dependent fashion and Ab3 was shown to be capable of attracting human sperm agglutination. These results indicate that anti-Id which may mimic an epitope of the 84 kD protein could be exploited as an antigen to raise antibodies against sperm protein.