Liver UDP-glucuronyltransferase activity towards bilirubin was studied in a total of 88 human subjects, including foetuses, premature and full-term newborn babies, infants, children, and adults. Determination of very low enzyme activity was performed by high-pressure liquid chromatography. Prenatal and postnatal changes of the activity can be divided into four developmental phases, i.e. middle foetal, late foetal, neonatal and early infantile, and mature. The activities of the first three phases corresponded to about 0.1, 0.1-1 and 1-100%, respectively, of the mature-phase values (mean +/- S.D.: 1320 +/- 514 microgram/h per g of liver, n = 27).