Short-chain fatty acids (SCFAs) have long been known to exert cellular effects on blood leukocytes. Acetate, propionate, and butyrate represent the most capable SCFA, inducing calcium mobilization which subsequently regulates leukocyte function in the immune system. We have cloned the previously described putative orphan G-protein coupled receptor, GPR43, and have functionally identified SCFA as the activating ligands. Acetate and propionate were found to be the two most potent ligands, although butyrate, formate, and valerate (in this order of potency) also were able to induce receptor activation. Both the human and mouse receptor homologues were found to share the same pattern of ligand activation. This finding, together with a high degree of amino acid sequence similarity between the mouse and human homologues, indicates an evolutionary conserved function. Upon ligand stimulation, the receptor mobilized intracellular calcium in both a recombinant system as well as in human granulocytes. We found the human gene to be predominantly expressed in peripheral blood leukocytes and, to a lesser extent, in spleen. We suggest the designation FFA(2)R to this second receptor activated by free fatty acids. The first-described FFAR, now named FFA(1)R, is activated by medium- to long-chain free fatty acids.