Treatment of rabbits with benzene (880 mg/kg/day), s.c. for 3 consecutive days, caused 3.8- and 5.7-fold increases in aniline 4-hydroxylation rates of liver and kidney microsomes, respectively. Benzene treatment markedly enhanced hydroxylation rats of p-nitrophenol by liver and kidney by 7.2- and 4.2-fold, respectively. Both of these enzymes are associated with cytochrome P-450 LM3a. In contrast, the activity of benzphetamine N-demethylase, associated with P-450 LM2, was not altered significantly in either liver or kidney microsomes. Although the total cytochrome P-450 contents of liver and kidney microsomes were not altered significantly by the benzene treatment, in the case of liver microsomes, formation of a new cytochrome P-450 with an apparent Mr of 51,400 was observed on SDS-PAGE. On the other hand, in the kidney microsomes, the intensity of the bands corresponding to approximate Mr of 50,000 and 51,400 was markedly increased. The results of the present work, in combination with those of the previous work (Arinç et al. 1988), indicate the existence of tissue specificity in the induction of rabbit P-450 isozyme by benzene.