In order to identify the possibility of prenatal or perinatal bacterial contact with immunization of the cellular immunity system as underlying cause of the "idiopathic" newborn icterus (without blood group incompatibility) the lymphocyte transformation test with addition of streptolysin O or E. coli antigen was carried out in 68 newborns with a birth weight ranging between 1260 and 4200 g. The sensitization rate identified among the newborns with hyperbilirubinaemia did not differ significantly from those of the control group. Thus an ensured connection between a prenatal streptococcus or E. coli contact and the appearance of an idiopathic newborn hyperbilirubinaemia could not be established.