Examination of the peroxidase isolated from the inkcap Basidiomycete Coprinus cinereus shows that the 42,000-dalton enzyme contains a protoheme IX prosthetic group. Reactivity assays and the electronic absorption spectra of native Coprinus peroxidase and several of its ligand complexes indicate that this enzyme has characteristics similar to those reported for horseradish peroxidase. In this paper, we characterize the H2O2-oxidized forms of Coprinus peroxidase compounds I, II, and III by electronic absorption and magnetic resonance spectroscopies. Electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) studies of this Coprinus peroxidase indicate the presence of high-spin Fe(III) in the native protein and a number of differences between the heme site of Coprinus peroxidase and horseradish peroxidase. Carbon-13 (of the ferrous CO adduct) and nitrogen-15 (of the cyanide complex) NMR studies together with proton NMR studies of the native and cyanide-complexed Coprinus peroxidase are consistent with coordination of a proximal histidine ligand. The EPR spectrum of the ferrous NO complex is also reported. Protein reconstitution with deuterated hemin has facilitated the assignment of the heme methyl resonances in the proton NMR spectrum.