Immunologically specific blocking factors (BF) are believed to play a protective role in the maintenance of pregnancy. We compared the levels of BF in normal pregnant women and in patients with recurrent spontaneous abortion (RSA) and observed that the two groups differ significantly from one another. MLC inhibitory activity was present throughout normal pregnancy and was found to be specific to the husband's HLA antigens. Immunotherapy was performed in 28 recurrent aborters using husband's lymphocytes, after which levels of blocking antibodies increased in 23 patients. The difference in the label of MLR BF in the preimmunotherapy and postimmunotherapy groups was statistically significant and was associated with successful pregnancy outcome in 82.15% of cases. The appearance of the MLR BF and its continuation in a successful pregnancy might represent an appropriate immune state that may be playing an important role in maintenance of pregnancy.