The incidence and identity of blood group antibodies among 4,653 gravid women and in about 9,000 male and 21,000 male and female recipients of blood transfusions were compared. Pregnant women included were all available obstetric patients referred for an attempt at universal prenatal serodiagnosis in a 5 year period and were unselected for their D (Rho) blood group. There were 3.8 per cent of gravid women, 0.92 per cent of males, and 1.3 per cent of all transfusion recipients who were isosensitized to blood troup antigens. This almost threefold increase in blood group antibody production by obstetric patients was not attributable only to transplacental D (Rho) sensitization common to them, as 0.9 per cent pregnant subjects and 0.39 per cent male transfusion candidates formed isoantibodies other than anti-D (anti-Rho). The D (Rho)-negative maternity patients produced predominantly anti-D or anti-C + D antibodies, but some also formed antibodies of anti-Lewis, anti-Kell, and anti-Duffy specificity. Within the group of D (Rho)-positive pregnancies, one mother produced anti-D, and others most often made anti-Lewis, anti-E, anti-c, anti-Kell, or anti-Duffy isoantibodies. All of these immunoglobulins among pregnant women were capable of inducing hemolytic transfusion reactions and some were responsible for instances of erythroblastosis due to maternal blood group sensitizations other than D (Rho), to be reported subsequently. No maternal transfusions were delayed while seeking compatible blood in the sensitized women and no hemolytic transfusion reactions occurred. It is suggested that the gravid woman is at greater risk for blood group sensitization due to transplacental immunizations from incompatible pregnancies, frequent challenges by red cell antigens in transfusions, and enhanced sensitization by the combined antigenic stimuli of incompatible transfusions followed by fetomaternal blood group incompatible pregnancies. The advantages of prenatal serodiagnosis are that compatible blood for the isosensitized mother and her infant is available promptly at delivery, unsuspected erythroblastosis is detected, and maternal hemolytic transfusion reactions are averted. The results suggest that prenatal serodiagnosis is optimal preventive medicine that should be prescribed by the obstetrician—in every pregnancy.