Lithocholic acid (LA) conjugates interact with M3 receptors, the muscarinic receptor subtype that modulates colon cancer cell proliferation. This observation prompted us to examine the action of bile acids on two human colon cancer cell lines: H508, which expresses M3 receptors, and SNU-C4, which does not. Cellular proliferation was determined using a colorimetric assay. Interaction with muscarinic receptors was determined by measuring inhibition of muscarinic radioligand binding and changes in cellular inositol phosphate (IP) formation. Lithocholyltaurine (LCT) caused a dose-dependent increase in H508 cell proliferation that was not observed in SNU-C4 cells. After a 6-day incubation with 300 microM LCT, H508 cell proliferation increased by 200% compared to control. Moreover, in H508 cells, LCT caused a dose-dependent inhibition of radioligand binding and an increase in IP formation. LCT did not alter the rate of apoptosis in H508 or SNU-C4 cells. These data indicate that, at concentrations achievable in the gut, LA derivatives interact with M3 muscarinic receptors on H508 human colon cancer cells, thereby causing an increase in IP formation and cell proliferation. This suggests a mechanism whereby alterations in intestinal bile acids may affect the growth of colon cancer cells.