The kinetic analysis of the enzyme UDP-glucuronosyltransferase (UDPGT) responsible for the conjugation of bilirubin, suggest that it is a multisubunit enzyme in which there is cooperative binding of substrate to the subunits. The binding of bilirubin to UDP-glucuronosyltransferase shows positive cooperativity with an apparent dissociation constant of 7.824 x 10(-4) +/- 6.405 x 10(-4) mM. The apparent Hill coefficient for bilirubin to UDPGT is 2.9. The binding of UDP-glucuronic acid exhibits kinetics with mixed cooperativity. Analysis with the Hill equation give an apparent dissociation constant of 6.873 +/- 3.816 mM and a Hill coefficient of 4.028 +/- 1.045. These values of the Hill coefficient are consistent with an enzyme being an oligomer with 6 subunits, since the actual number of subunits must be greater than the apparent Hill coefficient.