Carrageenans are seaweed extracts comprising high molecular weight sulphated polygalactosides. They are used in foods at concentrations of up to 2.5% as thickening and gelling agents. When degraded to lower molecular weight forms, they have been shown to induce ulcerative colitis and colon cancer in laboratory animals. Furthermore, undegraded carrageenan (CG) has been shown to promote azoxymethane and methylnitrosourea initiated carcinogenesis, but the promotion mechanism is unclear. To determine if this mechanism involves alterations of tissue drug-metabolizing enzyme system (DMES) activities, six groups of five guinea-pigs each were administered 0.2% kappa undegraded, 0.2% i undegraded, 1% kappa degraded or 1% i degraded CG, or control solutions in the drinking-water for 8 wk. Microsomal and cytosolic DMES activities of the liver, small intestine and colon were determined. The kappa undegraded CG group exhibited significant (P less than 0.05) increases in small intestine cytochrome P-450 levels and benzo[a]pyrene hydroxylase activities. These data suggest that undegraded CG may selectively induce DMES activities in the small intestine mucosa.