Employing subtype-specific antisera, we measured the relative immunoreactivity of five muscarinic acetylcholine receptor (mAChR) subtype proteins (m1-m5) in the human iris. The most intensive FITC immunofluorescence was detected by the anti-m3 antibody, followed by anti-m1 and -m5 antisera, in the iris sphincter muscle cells. Only very weak fluorescence was obtained by anti-m2 and -m4 antibodies. In dilator muscle cells, weak but not consistent immunoreactivity was found by anti-m1 and -m5 antibodies. The results suggest that the m3 muscarinic receptor is the predominant subtype in sphincter muscle cells of the human iris.