Endogenous bile acids such as chenodeoxycholic acid have been shown to display a suppressive effect in uitro on mononuclear cell activation. We investigated the signal transduction pathway involved in the effect of chenodeoxycholic acid on monocyte procoagulant activity, a model of monocyte activation. Chenodeoxycholic acid (25 to 250 pmol/L) had a concentrationdependent inhibitory effect on procoagulant activity expressed by endotoxin-stimulated mononuclear cells, with half-maximal and maximal inhibition occurring at concentrations of 100 and 250 pmol/L, respectively. The inhibitory effect of chenodeoxycholic acid was (a) closely mimicked by 4kphorbol 12p-myristate 13aacetate (PMA), a protein kinase C activator, but not by forskolin or dibutyryl cyclic AMP, two activators of the protein kinase A-dependent pathway; (b) prevented by staurosporine, a potent protein kinase C inhibitor; (c) partially abolished in protein kinase C-depleted cells; and (d) observed in conditions under which chenodeoxycholic acid, like PMA, significantly increased (41%) protein kinase C activity, as assessed by phosphorylation of exogenous (histone III-S) and endogenous (37-kD protein) substrates. In conclusion, our results (a) provide clear evidence of a marked inhibitory effect of chenodeoxycholic acid on monocyte activation, suggesting a potential role of primary endogenous bile acids in the immune defect associated with cholestasis; and (b) indicate that the inhibition of monocyte activation by chenodeoxycholic acid is mediated by way of protein kinase C activation (HEPATOLOGY 1994; 19: 1164-1170.) Bile acids are endogenous compounds with wellknown physiological properties including both cholesterol homeostasis and bile formation. Recently it was shown that bile acids behave as immunosuppressive compounds. In vitro, we have found that chenodeoxycholic acid (CDCA), one of the two major naturally occurring human bile acids, was able to inhibit, at least in part, the mixed lymphocyte reaction (1). CDCA also