Lactoperoxidase (LPO) is the enzyme of first-line defense in lung mucous, milk and saliva; oxidizing potential invading bacteria and viruses with peroxide and halide/thiocyanide substrates. The heme is covalently bound by ester groups to heme methylenes at positions 1 and 5, so is both functionally and structurally related to intracellular, mammalian peroxidases. LPO was isolated from cow's milk, concentrated to over 2 mM heme, reduced with dithionite under CO atmosphere. FTIR spectra were collected at 1 cm-1 resolution in a 0.2 mm ZnS, thermostated cell at 25C, using D2O, 50 mM buffer, 1 mM EDTA from pD 3.0 to 8.8, covering the range of highest enzymic activities. Two CO stretch absorbances were observed in all buffers, with varying area ratios at 1942 and 1955 cm-1 maxima. The spectral bands were separated using a curve resolving program fitting to the Voight approximation; the band widths at half peak maximum were calculated between 11 and 13 cm -1. The stretch maxima are significantly higher energy than observed for plant peroxidases but the peak widths correlate to the Fe(III)/(II) reduction potential at pH 7, generality. Molecular orbital calculations of a model heme active site address heme-asymmetry, isotopic shifts of vibrational bands and the free electron character of porphyrin electron states.