Foetal erythrocytes have an enhanced resistance to osmotic haemolysis that they retain for the first 5 days after birth, particularly in premature newborns. The erythrocyte fragility test was used to study the increased resistance to osmotic haemolysis in 155 healthy newborns (50 NGA born to term, 55 NGA premature and 50 SGA born to term) as well as 31 newborns (20 born to term and 11 premature) with aspecific hyperbilirubinaemia. The drug used was glycerol which is specific in its reaction to alterations in erythrocyte membrane stability and made it possible to assess changes arising in the first month after birth, both naturally and in response to phototherapy. The results show a much higher resistance in neonatal than adult erythrocytes that gradually decrease during the first month after birth without, however, falling to adult levels. No changes in resistance attributable to the phototherapy adopted in newborns with hyperbilirubinaemia were encountered. The glycerol test proved extremely sensitive in the diagnosis of neonatal haemolytic anaemias easy to use so that congenital spherocytosis can be identified earlier than is otherwise possible.