IgG subclasses of anti-Rh antibodies in the sera of 65 pregnant women were determined. The frequency of subclasses was: IgG1-87.6%, IgG2-23.0%, IgG3-56.9% IgG4-7.7%. In different cases of Rh incompatibility, no correlation was found between the intensity of the hemolytic disease of the newborn and the pattern of IgG subclasses in maternal antibodies. On the basis of the whole material analysis it was observed, however, that the more pregnancies with fetomaternal incompatibility the women had and the more severe the course of the hemolytic disease was in their babies, the more frequently the IgG3 sublcass was present in the pattern of antibodies and the less frequent were the IgG2 and IgG4 subclasses. We have put forward the hypothesis that at the time of strong stimulation of the immunological mechanisms, antibodies are produced by many cell clones and that their activity may decline and disappear with progressing development of clones specifically producing anti-Rh antibodies of the subclasses IgG1 and IgG3.