Thirty-two primary habitual aborters who had more than 3 abortions during the early stages of pregnancies without any known causes of habitual abortions were studied. The degree of HLA-DR compatibility of the patient couples was similar to that of 70 control couples. The patients were treated with intra-dermal injections of their husbands' lymphocytes 4 times every 2 weeks. The skin reactions at the sites of the injection were estimated 24 hours and 48 hours after the first and fourth injections. The mixed lymphocyte reaction (MLR) after the last injection decreased in the patients whose skin reaction was weaker following the last injection than the first (p = 0.007), and these patients had a good prognosis for subsequent pregnancies (p = 0.010). On the other hand, the MLR after the last injection did not weaken in the patients whose skin reaction did not become weaker (p = 0.007), and in these patients abortion resulted in subsequent pregnancies (p = 0.010). The HLA-DR compatibility of the patient couple and anti-HLA-DR antibody induced by the treatment did not affect the prognosis for subsequent pregnancies.