A number of factors were analyzed for their predictive value in indicating the severity of haemolytic disease of the newborn (HDN) in 72 infants. The factors investigated were: maternal antibody titre in the indirect antiglobulin test, the number of antibody molecules on sensitized standard red cells evaluated by a radiometric antiglobulin test, the IgG subclass specificity and the reactivity in monocyte-monolayer assay (MMA) and in the rosette assays with lymphocytes and granulocytes from healthy individuals. The results of the MMA correlate much better with the severity of HDN than the antibody titre. In clinically unaffected infants the reactivity in the MMA never exceeded 20%, while in the severe/very severe group it was always greater than 20% (in 95% of very severe cases even above 50%). The number of IgG-bound molecules was also shown to closely correlate with the clinical severity and there was a much greater proportion of severe/very severe cases exhibiting combined IgG1 and IgG3 specificity. Of all the evaluations performed the rosette assays with lymphocytes and granulocytes were found to be less useful in predicting the severity of HDN.