The foetal trophoblast behaves as a homograft in the maternal host, yet it survives. This referes to modulation of maternal immune system to antigeni-cally foreign foetus and placenta. Vari­ous investigations demonstrated slight depression of maternal immune sys­tem during pregnancy, but the mother does not develop specific immunologi-cal tolerance to the foetus. The de­pression was expressed by increased number of suppressor cells (Moore et al.,1983), reduced activity of natural killer (Gall, 1983). Rocklin et al. (1976) demonstrate the prescence of immunologic block­ing factor in the sera of normal preg­nant multigravida women. This factor is absent from the sera of untrans- fused multipara (Stewart et al., 1984). It appears in the maternal sera of both primi and multigravidas, as early as the 4tJi - 5th week of gestation, its activity increased with longer gesta­tion to reach maximum, at the time of delivary, then it begin to disappear (Kasakura, 1971). Johnson et al. (1984) claimed absence of the block­ing factor in women suffering from ha­bitual abortion. To our knowladge the pathological changes in placenta in patients with decreased blocking factor is not de­scribed in the work English literature, so the present work is planned to study the pathological changes in the products of conception in patients with deficient blocking factor.