Placentae were examined from 120 women whose pregnancy and delivery was normal, from 264 women whose pregnancy or delivery was complicated, and from 98 women who were Rh-negative without antibodies and 35 women Rh-negative with antibodies. The presence of Kline's haemorrhages, intervillous thrombi, infarcts, and retroplacental haemorrhages was positively correlated with the presence of foetal cells in the maternal circulation. When there were no maternal antibodies transplacental haemorrhages occurred occasionally in the absence of such placental lesions but more frequently when these lesions were present. Moreover, the greater the number of lesions in a placenta the greater the size of the transplacental haemorrhage. In Rh-negative women with antibodies the observed incidence of transplacental haemorrhage was significantly less despite an appreciable increase in placental lesions.