Anti-D haemolytic antibody concentration and chemiluminescence (CLT) opsonic index was measured in maternal blood obtained from 20 alloimmunized pregnancies at 17-28 weeks undergoing intrauterine fetal blood sampling for the estimation of fetal haemoglobin concentration. The fetal haemoglobin concentration was significantly associated with the maternal serum CLT opsonic index (r = -0.566, P < 0.01) but not with the maternal anti-D concentration (r = -0.329). The data of this study indicate that measurement of maternal serum CLT opsonic index may be more accurate than anti-D quantification in providing non-invasive prediction of the degree of fetal anaemia.