Mrs P. presented at 13 weeks of gestation with apparent anti-C+D. At week 34, with antibody levels of 168 IU/ml, a D-negative (r'r) baby was delivered with a strongly positive DAT and an Hb of 3.0 g/dl. Anti-G in maternal serum was isolated by adsorption and elution from R2R2 cells and shown, using flow-cytometric and chemiluminescence assays, to sensitize r'r cells at levels of cell-bound IgG consistent with fetal haemolysis. In an analysis of 28 sera from alloimmunized women with over 5 IU/ml anti-C+D, 2 sera were shown to contain levels of anti-G consistent with moderate or severe haemolytic disease of the newborn (HDN). Thus HDN due to anti-G may not be rare. An analysis of 187,037 blood donors in the south-west of England showed the r' gene frequency to be 0.005897 suggesting that approximately 2.9% of matings of rr women with D-negative fathers can produce an r'r baby. These findings highlight the need for the continuous non-invasive monitoring of D-negative fetuses of women with apparent anti-C+D.