Usability of repeated liver biopsies was examined for the estimation of drug residual level in the calf liver. The liver biopsy was carried out under ultrasonic observation. Twenty four calves were given 20, 000 units of benzylpenicillin (PC-G) per kg body weight. Twelve calves were biopsied and sacrificed within 2 hrs after the biopsy at 3 and 7 days after the administration, and 7 calves were sacrificed at 3, 7, 14 and 21 days after the administration to take the liver samples (the serial sacrifice group). Five calves were biopsied repeatedly 15 times during 21 days following the administration to obtain the liver parenchyma (the repeated biopsy group). There was no significant difference (p=0.05) in PC-G concentrations between the biopsy samples and the samples obtained by sacrifice immediately after the biopsy, nor between the samples of "the repeated biopsy group" at 3 and 7 days after the administration and those of "the serial sacrifice group" at the corresponding points. Pharmacokinetic parameters and the decay lines calculated from the mean concentrations in both groups were almost identical. These results suggest that the estimation of drug residual level in the calf liver taken by repeated biopsies in place of the serial sacrifice is useful to determine the withdrawal time of drugs remaining predominantly in the liver.