PROBLEM: The questions of whether production of mixed lymphocyte reaction‐blocking factors (MLR‐BFs) after immunotherapy with lymphocytes for recurrent spontaneous abortion (RSA) has prognostic value and whether cytotoxic antibodies are also involved were tested. METHOD OF STUDY: A prospective study with 33 patients who had a history of two or more abortions, lacking MLR‐BFs, was carried out. The patients received immunizations with lymphocytes and 6 weeks or later were tested for seroconversion of MLR‐BFs. Seventeen of these thirty‐three patients were evaluated for antipaternal cytotoxic antibodies. The results were correlated with the outcome of the next pregnancy after treatment. RESULTS: Eighty percent of the 33 patients had a live child. Of those patients having success, only 50% produced MLR‐BFs. Of those patients having a new loss, five did and two did not produce MLR‐BF ( P > 0.05). Regarding the 17 patients tested for cytotoxic antibodies, 4 of the 5 patients who tested positive had a new abortion, whereas only 1 of 12 whose tests remained negative did not have gestational success ( P < 0.01). CONCLUSION: The presence of MLR‐BFs is not a prognostic criterium for the outcome of pregnancy after alloimmunotherapy, and, consequently, it is not a good diagnostic tool for RSA of alloimmune cause.