The alpha and beta chains were prepared from canine hemoglobin by a modification of the method of Bucci and Fronticelli ((1965) J. Biol. Chem. 240, PC551-552) which involved treatment of hemoglobin with an excess of p-chloromercuribenzoate (pCMB), separation of the mercurated chains by chromatography on a DE-32 column with a salt gradient at pH 8.6, and regeneration of sulfhydryl groups of the chains with 2-mercaptoethanol. The SH titer was two per heme for both the regenerated alpha and beta chains. The titer decreased to four per tetramer of hemoglobin after equimolar recombination of both chains. Measurements of the absorption spectrum and oxygen binding showed that the chains were in a native state.