SUMMARYTo elucidate the etiology of icterus neonatorum which is still obscure urobilinogen tolerance tests were performed on 31 newborn and a few infants with diseases of the liver and bile passages. The technic of WITH (1946) was used. The results of the investigation point towards the existence of some degree of liver insufficiency in the newborn, but they are difficult to evaluate as the test is also influenced by other factors than the liver function, presumably the kidney function. The test is consequently of no use as a functional liver test in infants.Attention is drawn to the fact that the serum bilirubin concentration in icterus neonatorum is much higher than in the forms of hemolytic jaundice in adults, which speaks against the hemolytical theory of the origin of icterus neonatorum.