Activity of bilirubin glucuronyl transferase in mouse liver homogenates is low at birth, reaches a peak at 14 days of age, 2–3 times greater than adult values. Kinetic studies showed marked differences between microsomes derived from adult and 14-day-old animals. Michaelis-Menten constants (Km) were 5.9 × 10––5 m and 4 × 10––4 m respectively at these two ages. Km values increased from 5.9 × 10––5 m to 6.7 × 10––4 m when enzyme derived from adult animals were solubilized with deoxycholate. In contrast, solubilization did not modify the Km values in preparations from 14-day-old mice. The newborn Km value for unsolubilized enzyme was 8.8 × 10––5 m and 6.7 × 10––4 m for the solubilized preparation. Pretreatment of pregnant animals with barbiturates to induce enzymic activity was necessary for the kinetic studies in the newborn. These findings indicate that different forms of the enzyme exist at the several stages of development studied. Alterations produced by solubilization procedures suggest a common structural form which undergoes alteration during maturation.