In 80 newborn infants ABO-incompatible with their mothers, the lysis-inducing effect of the maternal IgG anti-A or anti-B antibodies in an antibody-dependent cell-mediated cytotoxicity (ADCC) assay and the antigen density of A or B antigens on the red cells of the children were measured. On the basis of the results, the children were divided into two groups--24 children in whom increased haemolysis was to be expected, and 56 children in whom it was not. Signs of haemolysis and serological features of ABO haemolytic disease of the newborn (ABO-HDN) were compared in these two groups and a control group of 120 ABO-compatible infants. The effect of the maternal antibodies in the ADCC assay, the titres of maternal IgG anti-A or anti-B antibodies, the results of the direct antiglobulin test on the red cells in the cord blood, and the titre of IgG anti-A or anti-B antibodies in the serum of the infants were compared for their ability to predict the severity of ABO-HDN. This was also done for the combination of the ADCC assay results plus the A or B antigen density and the direct antiglobulin test plus the titre of maternal IgG anti-A or anti-B antibodies. The ADCC assay with maternal serum was the most sensitive assay to predict ABO-HDN, and the combination of the ADCC assay with A or B antigen density determination the most specific test.