Two isoforms of UDP-glucuronyltransferase purified from rat liver (named GT-1) and kidney (named GT-2) have various properties in common but differ in their NH₂-terminal sequences. In this study, the two forms were further found to have common immuno-chemical properties, i.e., they could not be distinguished by Ouchterlony double diffusion and immunoblotting analyses. These isoforms also had the same inducibility as shown by immunoblotting analysis: GT-2 protein in rat was increased by treatment with β-naphtho-fiavone and 3-methylcholanthrene, whereas GT-1 was inducible by 3-methylcholanthrene. However, the effects of phospholipids on these enzymes were extremely different. 1-Naphthol glucuronizing activity of GT-1 was increased 7.5-8-fold by lysophosphatidyl-choline, but the activity of GT-2 was increased only 3-3.6-fold. The transferase activity of GT-1 toward 4-methylumbelliferone was increased 2-2.5-fold by dilauroylphosphatidyl-choline, but that of GT-2 was reduced, while its 4-nitrophenol glucuronidation activity was increased 1.5-fold by the phospholipid. These results indicate that the two similar UDP-glucuronyltransferases from rat liver and kidney interact differently with phos-pholipids and that the activation level of UDP-glucuronyltransferase activity with phos-pholipids depends on the aglycone substrates.