Summary.The majority of Rh(D) negative immunised mothers have some saline agglutinating anti‐D in their sera during subsequent pregnancies. This saline antibody is related to a complex effect on the severity of haemolytic disease of the foetus: the stillbirth rate is reduced while the severity of disease among liveborn infants increases slightly with rising titres of saline anti‐D. Also, when the observed saline anti‐D titre is I the severity of disease is markedly incresed while the protective effect of saline anti‐D on the stillbirth rate fails to operate. The significance of this is discussed.Mothers of group A and especially group B are more liable to form saline anti‐D, while the severity of HDN increases if the mother is of grou