SUMMARYCBA/J (H‐2k) females, mated with DBA/2 J (H‐2d) or DBA/1 J (H‐2q) males, exhibit a high rate of fetal resorption. In contrast, when H‐2 identical CBA substrains (i.e. CBA/Ca and CBA/N) are used, this phenomenon is not observed. On the other hand, before mating with DBA/2 J males, pre‐immunization of CBA/J females with spleen cells coming from BALB/c J or (DBA/2 x BALB/c J) F1 males (and not from other H‐2didentical males whatever their Mls alleles) has significantly decreased the fetal resorption rate. Thus, immunization against determinants other than classical H‐2d(K, I, D, L) antigens (transmitted as a dominant character and different from Mls determinants) can elicit anti‐abortive effects. Furthermore, it was observed that the spleen cell endowed with the anti‐abortive effects was neither a T nor a B lymphocyte. In contrast, peritoneal cells were able to reproduce the phenomenon, indicating that it may be mediated by a cell of the macrophage‐monocyte lineage. Finally, a first gestation was substituted for allo‐immunization of CBA/J females. The anti‐abortive effects of a first pregnancy by BALB/c J males (and not by other H‐2ksyngeneic or H‐2dallogeneic males) was observed in the course of a second pregnancy sired by DBA/2 J males. These data can be interpreted in terms of maternal recognition of an antigen present on both macrophages and trophoblast cells and necessary for a successful gestation, which is coded for by genes out
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