Plant foods contain nutritive and minor, nonnutritive components capable of inhibiting experimental carcinogenesis. Many of these cancer-protective extracts act by enhancing the activities of enzymes that can detoxify reactive substances. In the present study an extract of the spice plant rosemary was fed at concentrations of 0.3% and 0.6% (by weight) for 4 weeks to female A/J mice prior to determination of the activities of the detoxification enzymes glutathione-S-transferase (GST) and NAD(P)H: quinone reductase (QR) in lung, liver and stomach. Liver activities of GST and QR, and stomach GST activity were significantly increased in animals fed diets containing rosemary extract. However, diets supplemented with rosemary extract did not affect lung GST and QR activities. These results indicate that components of rosemary extract have the potential to protect mouse liver and stomach from carcinogenic or toxic agents.