This was a pilot study to investigate the possible roles of human lymphocyte antigen (HLA), antipeternal lymphocytotoxic antibodies, and maternal antipaternal mixed lymphocyte reaction (MLR) blocking antibodies in the maintenance of pregnancy following paternal leukocyte immunization for patients with recurrent abortions. A total of 36 patients with unexplained, first trimester, primary recurrent spontenous abortions were investigated for the detection of these two antibodies. There was a 43.3% rate of discordance in the presence of the two antibodies (P <0.05). A total of 26 of these women who lacked either antibody were immunized with paternal leukocyctes on two occasions and the assays were repeated post-immunization. The seroconversion rate was 50% for lymphocytotoxic antibodies and 61.5% for maternal serum (MLR) blocking antibodies. Twenty patients achieved pregnancies post-immunization, 11 completed their pregnancies successfully, and 9 re-aborted. A total of 83.3% of patients who developed MLR blocking antibodies post-immunization had successful pregnancies while those who failed to seroconvert aborted again. This difference is statistically significant (P<.05). A total of 50% of patients who developed lymphocytotoxic antibodies post-immunization had successful pregnancies while only 40% who failed to seroconvert re-aborted. This difference, we felt, was not statistically significant. The development of MLR blocking antibodies psot-immunization is a better indicator of subsequent successful pregnancies than lymphocytotoxic antibodies.