Using gel filtration and ultracentrifugation, the quaternary structure of muscle creatine kinase (isoenzyme MM) has been shown to depend on pH, protein concentration and ionic strength of solution. In solution the enzyme can exist not only as a dimer, but also as a monomer and tetramer with molecular weights of 40 000 and 160 000, respectively. High dilution of the protein solution and pH 6.0--5.0 provide for the monomer formation; the dimer can arise under various conditions, while the tetramer requires pH 9.0 and high protein concentration. The monomer differs from the dimer by a higher enzymatic activity, lability and needs thiol to maintain its catalytic activity. Dissociation of creatine kinase into more active subunits accounts for the increase in specific activity induced by protein dilution. It is assumed that dissociation can have physiological significance as one of the mechanisms of creatine kinase activation in muscles.