meso-Tetrakis(alpha,alpha,alpha,alpha-o-(1-methylcyclohexanamido)phenyl) porphinatoiron (III) bearing a proximal histidyl group at the beta-pyrrolic position via an acyl bond (4c) has been synthesized. Human serum albumin (HSA) incorporating the ferrous complex (4d) formed a stable O-2 adduct under physiological conditions (pH 7.4, 37 degrees C). Although an electron-withdrawing acyl group is attached to the porphyrin periphery, the O-2-binding affinity of HSA-4d was slightly higher than that of a similar analogue with a histidyl-alkylene group (2d).