SUMMARYImmunosLipprcssivc blocking factors in maternal serum arc usually determined by inhibition of mixed lymphocyte cultures (MLC), but reports on the importance of these factors for successful pregnancy are conflicting. Here we measured serum blocking activity in men. non-pregnant nullipiirous women, non-pregnant multiparous women, women with normal pregnancies, and in women who had had recurrent spontaneous abortions and were treated with leucocyte immunizations. Three ditferent equations were used for calculation of blocking activity: blocking effect index (BE!): stimulation index (SI); and blocking index (BI). By all three methods of calculation, significantly lower levels of blocking aetivity were noted for men and women compared with pregnant women and muttiparae. In the patients with a history ol' recurrent spontaneous abortions blocking activity as determined by BEI and BI increased into the positive range after treatment with infusions of third-party donor leucocytes in a statistically significant number of women {P<0-05). However. blocking activity as determined by BEI had a higher predictive value for successful pregnancy than did ihat determined by BI or SI. Our data suggest that Ihe equation used for ealculaling BEI is superior to other methods for the determination of blocking activity when monitoring the response to leucocyte immunization in women with recurrent spontaneous abortion. However, these results also cast doubt on the importance of blocking antibodies in histories of recurrent abortion, since pregnancies occurred in the absence and spontaneous abortions occurred in the presenee of blocking activity.