Changes in lymphocyte subsets in whole blood were analyzed sequentially by flow cytometry with an automated leukocyte differential system in 15 patients with unexplained recurrent spontaneous abortions, each of whom underwent vaccination(s) with her husband's lymphocytes. Mitogen responses of peripheral blood lymphocytes (PBL) were also examined in these patients. The reactivity of PBL against mitogens revealed no significant change in each patient before and after vaccination(s) with her husband's lymphocytes. The CD4:8 ratio was observed to decrease significantly during 22 and 28 days after the first vaccination with a significant increase in the percentage of T suppressor-cytotoxic (CD8) cells. This change was also observed after the second vaccination. The percentages of other subsets did not change significantly after vaccination(s). In 11 patients out of 15, the pregnancy continued successfully and correlated with a predominance of Ts/c (CD8) over TH/I (CD4) cells in the first trimester. These changes in lymphocyte subsets may indicate the induction of immune enhancing mechanisms and it is suggested that continuation of the predominance of Ts/c cells induced by immunotherapy might be important for the successful maintenance of pregnancy.