The ability of soy to induce phase II detoxification enzymes was evaluated in male Sprague-Dawley rats. Soy-beans contain biologically active compounds that are known inducers of phase H enzyme activity. Rats were fed soy flour (SF) or soy protein isolate (SPI) to provide 75% of total protein as soy. Rats were given free access to food for one-and two-week periods before enzyme activity was compared with that of casein control groups (AIN- 93G). Hepatic glulathione S-transferase (GST) activity was significantly greater in rats fed SF for one and two weeks and in rats fed SPI for two weeks than in controls. Quinone reductase activity was significantly greater (12- to 14-fold) in the colon of rats fed SF and SPI for two weeks and in serum (1.8- to 2-fold) in the SF group at one and two weeks. Liver, kidney, and small intestine uridine 5'-diphosphate-glucuronosyl transferase activity was significantly increased in the SPI and SF groups at two weeks. A time dependence in induction of phase II enzymes was observed in several tissues. There was no significant difference in total liver glutathione in either diet group compared with controls. The data indicate that dietary, soy enhances phase II enzyme activity, especially quinone reductase and uridine 5'- diphosphate-glucuronosyl transferase, which could lead to protection from potentially harmful xenobiotics.