Reported here is the first example of a partial D antigen stimulating the production of anti-D: stimulation was of fetal origin. During her second pregnancy, anti-D developed in the serum of a D-negative mother who had received Rh immunoglobulin after the birth of her first D-positive child. Her second baby had moderate neonatal jaundice and was successfully treated by phototherapy. Subsequently the red cells of the father and of the first child were shown to carry a partial D antigen of category DVa type. Six available batches of Rh immunoglobulin reacted with DVa cells.