HLA antigens are present on immature red cells (Harris & Zervas 1969), but disappear from their surface when they become mature and enter into the circulation (Silvestre et al. 1970). Nevertheless some remnants of HLA substance may survive this antigenic switch and possibly persist on adult red cells (Morton et al. 1969, 1971, Doughty et al. 1973). A shortened survival of HLA incompatible reticulocytes has been reported when they are transfused into recipients with circulating anti-HLA antibodies (Zervas et al. 1972). As to anti-HLA antibodies possibly reacting with erythrocytes in cases of fetomaternal immunization, Moulinier (1970) claimed that they would potentiate any hemolytic process in newborns of Rh-immunized women, but successive investigations failed to confirm this (Ahrons & Glavind-Kristensen 1971, Nymand 1975). Also, Nymand (1975) found no association between lymphocytotoxic antibodies in mothers and high bilirubin levels in the umbilical cord blood collected at delivery. On the contrary, when the onset of neonatal jaundice in the postpartum offspring was monitored, newborns of mothers with lymphocytotoxins appeared to become jaundiced more frequently than expected (Reekers et al. 1975). The present study has been undertaken to investigate the relationships between complement dependent lymphocytotoxins (CdL) in the mothers and the bilirubin levels in the newborns during the first days of life.