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A method of isolation of the isoenzyme BB of creatine kinase from ox stomach is described taking into account its high instability and the presence of notable amounts of tropomyosin in vertebrate smooth muscle extracts of low ionic strength. The molecular parameters of the enzyme isolated from ox stomach do not differ from those of the creatine kinases MM and BB of vertebrates. The amino acid composition and the peptide maps reveals significant differences from that of the ox brain isoenzyme BB.