The effects of alloantisera on lymphocyte proliferation were studied in 45 multipara. Lymphocytes from mother (M), father (F) and unrelated person (U) were typed for HL–A specificities and cultured in bilateral and unilateral MLC comparatively in mother's and father's sera. Statistical analysis of these paired cultures demonstrated an inhibitory effect of mother's sera in 16 cases. Blocking factors were directed against stimulator as well as against responding lymphocytes. In most cases this effect was observed in the absence of detectable cytotoxic antibodies. PHA response was but slightly affected. The blocking effect was often directed against unrelated lymphocytes, which did not share evident HL–A specificities with the father. The possible heterogeneity of antibodies blocking the MLC and the diversity of their mode of action are outli