We determined clinically whether a killed streptococcal preparation (KSP), a biological response modifier, is as effective as paternal lymphocyte immunotherapy for unexplained recurrent pregnancy abortion (RSA) therapy. The success rate of adverse pregnancy in the study group of 23 RSA cases, who were administered low doses of KSP before and during early pregnancy, was statistically compared with that in a control group of 205 women who received paternal lymphocyte immunotherapy. The association of natural killer (NK) cell activity in the peripheral blood with pregnancy outcome was also assessed. The success rate in the study group was 73.9% (17/23), compared to 75.1% (154/ 205) observed for the controls (not significant). Most of the successful cases exhibited low levels of NK cell activity in the peripheral blood. Immunotherapy with low doses of KSP is as effective as that with paternal lymphocytes, providing a simple and safe alternative therapy for unexplained RSA. Suppression of NK cell activity by some immunoregulatory mechanism was also found to have potential benefit in terms of a successful pregnancy outcome.