Free bilirubin (FB) and total bilirubin (TB) were determined in 154 samples of blood taken from 112 jaundiced newborns: 51 prematures without hemolysis (19 of these with RDS); 26 full terms presenting AB0 incompatibility; 35 newborns (both prematures and full terms) presenting rhesus incompatibility. Kernicterus was observed in seven cases and only three occurred in the TB group above 20 mg/100 ml; 57 cases had FB equal to or above 0.1 mg/100 ml and all kernicterus fell into this category. In the other 55 cases in which FB was less than 0.1 mg/100 ml no kernicterus was observed. In the group of healthy fullterm newborns presenting AB0 incompatibility, 15 had FB above or equal to 0.1 mg/100 ml ranging between 0.1 and 0.4 mg/100 ml; however no kernicterus was observed during the neonatal period. On the contrary, in the group of prematures a little more than half of the cases had a FB ranging from 0.1 to 0.4 mg/100 ml whereas four macroscopic kernicterus cases were observed. The difference between the two groups compels us to consider other factors than those acting on the albumin-bilirubin binding especially those acting on the blood-brain barrier and on the fixation of the pigment by the neurons. A second series of 605 consecutive autopsies, on a period of 10 years, on prematures excluding light-for-dates and cases of hemolysis, evaluates the distribution of 40 kernicterus as a function of birth weight. On this second series kernicterus appears with maximal frequency for birth weight between 1,000 and 1,250 g, with a nonnegligible frequency at 1,500 to 2,000 g and was absent between 2,000 and 2,500 g. 13 kernicterus were observed for a peak TB below 12 mg/100 ml. These results seem to justify a systematic determination of FB in the premature weighing less than 2,000 g from the 24th hour of life whether he has jaundice or not.