The relationship between surfactant synthesis measured by amniotic fluid lecithin concentration and L/S ratio, and the degree of intrauterine haemolysis as judged by the Liley optical density difference, has been studied in 125 samples from 62 rhesus immunized pregnant women. In forty cases where the samples were obtained within one week of delivery, the results have been correlated with cord blood haemoglobin concentrations. There is evidence that a mild intrauterine haemolytic process may stimulate surfactant production while severe intrauterine haemolysis may be inhibitory in some cases. Intrauterine anaemia did not seem to have any effect upon respiratory function after birth as long as the L/S ratio was adequate.