The importance of glucose-6-phosphate dehydrogenase deficiency in the causation of severe neonatal jaundice in Israel was investigated. Communities with very high frequencies of enzyme deficiency and those in which it was very rare did not differ significantly in the frequency of severe neonatal jaundice not due to iso-immunization. However, in one investigated sample of Iraqi Jews the number of enzyme-deficient cases among the jaundiced infants was significantly higher than expected from the gene frequency, suggesting that some cases of haemolytic disease of newborn due to enzyme deficiency did occur in this group. With one exception all the cases were mild and, did not require exchage transfusion. The overall results suggest that although a few such cases may occur in this country, glucose-6- phosphate dehydrogenase deficiency cannot be considered an important aetiological factor in the causation of severe haemolytic neonatal jaundice in Israel, in spite of the existence of population groups with a very high frequency of the enzyme deficiency. The possible causes of differences between the findings in Israel and those reported from Greece, Sardinia and Malaya are discussed.