Activation of the formyl peptide chemoattractant receptor (FPCR) of phagocytic cells mobilizes intracellular calcium stores and affects the plasma membrane potential. Affinity crosslinking of FPCR has demonstrated a 60-80 kDa glycoprotein, with core peptide of 32 kDa. It is not known whether functional FPCR is this single peptide or requires multiple subunits. We used Xenopus oocyte expression system to determine the size of mRNA required for synthesis of functional FPCR. Injection of oocytes with poly(A)+ RNA from HL60 cells differentiated to the granulocyte phenotype resulted in acquisition of formyl peptide-specific responses (inward transmembrane current with a reversal potential consistent with a chloride conductance, and calcium efflux). FPCR activity expressed in oocytes had a ligand concentration dependence, ligand structure dependence and pertussis toxin sensitivity similar to those reported in phagocytic cells. When RNA was size fractionated, a single peak of FPCR activity at 2 kilobases was observed after injection of mRNA into oocytes. Our data strongly suggest that FPCR is composed of a single-sized polypeptide.