ABSTRACT: In this study we have compared the ability of two assay systems, erythrocyte antibody rosette inhibition (EAI) and cellular enzyme-linked immunospecific assay (CELISA) to detect maternal alloantibody activity during pregnancy. Antibody activity to antigens on paternal lymphocytes was present in nine of 23 primigravid sera tested by EAI and in seven of 23 by CELISA. In multiparous sera, antibodies directed to paternal lymphocytes were detected in 11 of 15 individuals by EAI and in six of 15 by CELISA. The techniques correlated significantly when assaying the humoral response in sera from multiparous women. The lack of correlation when assaying primigravid sera suggests that both assays encounter difficulty in detecting the low titres of antibodies present.