These case is described of a woman with repeated stillbirths. Her serum contained high-titered anti-M antibodies. Treatment of the serum with 2-mercaptoethanol resulted in do reduction in anti-M titer. The serum fraction consisting mainly of IgG exhibited a markedly high anti-M titer compared with the IgM fraction. Anti-M antibodies eluted from OM red cells could be removed by precipitation with specific anti-IgG. Thus, the repeated stillbirths are ascribed to IgG and anti-M antibodies were formed during incompatible pregnancy and crossed the placenta.