H-1-C-13 HETCOR studies were performed on some low-spin heme-containing systems. From the available data or our extended proton assignment, complete carbon assignments were performed. The carbon (and proton, when not yet already performed) chemical shifts were factored out into metal-centered pseudocontact shifts and contact (plus ligand-centered pseudocontact) shifts. The ratios between proton and carbon contact shifts were found to be constant for the symmetric bis(imidazole) protoporphyrin and to vary for the asymmetric protein systems. This feature is discussed also in the light of the data already available on other heme proteins.