It was demonstrated that the inbred strain EHBR had the C phenotype of esterase-3 judging from the absence of liver microsomal beta-glucuronidase and the pattern of esterase activities of liver homogenates after analytical isoelectric focusing. In addition, in the strain EHBR, liver microsomal hydrolase activities of acetanilide and isocarboxazid which are hydrolyzed well by esterase-3 were lower than in outbred Sprague-Dawley rat and inbred LEW rat having the D phenotype of esterase-3. These results suggest that the phenotype difference of esterase-3 is possible to cause the strain differences of liver microsomal carboxylesterase activities.