Hepatic bilirubin (Bil-GT) and paranitrophenol glucuronyl transferase (PNP-GT) activities were measured in 26 subjects with Gilbert's syndrome (GS) and in one subject with a Crigler-Najjar type 2. Firstly, the results allowed us to distinguish three groups of patients in GS. In the first group, Bil-GT activity decreased by 25% of that of the controls and PNP-GT activity was normal. In the second group, Bil-GT decreased by 25% of that of the controls and PNP-GT decreased by 50% of that of the controls. In the third group, Bil-GT decreased by 50% of that of the controls and PNP-GT activity was normal. Secondly, the results showed in the Crigler-Najjar type 2 that Bil-GT activity was 25% of that of the controls and PNP-GT was 10% of that of the controls. From these results the following hypothesis has been raised: (1) the subjects of the third GS group were probably heterozygous to the homozygous disease which affected the subjects of the first GS group, and (2) the subjects of the second GS group were most likely heterozygous to the homozygous disease which affected our Crigler-Najjar type 2. However, in the present state of our knowledge, the scheme of GS classification which we propose requires confirmation.