The action exerted by selective sensitizations to alloantigens coded by the Major Histocompatibility System of the mouse, H-2, on the phagocytic ability of the mouse peritoneal macrophages has been studied. The results suggest that when immunizations are performed between completely incompatible animals for the H-2k haplotype, after the third immunization the total number of macrophages able to carry out phagocytosis decreases very significantly. When the incompatibilities, however, are due to the left half of the H-2 system or to minor histocompatibility antigens, such an inhibition does not appear.