Anti-phosphorylase produced in cocks against phosphorylase from rabbit skeletal muscle inhibits phosphorylases isolated from mammalian heart and skeletal muscle to different degrees. On the basis of the differential inhibition observed an immunotitration method was developed to determine isophosphorylases in crude heart extracts. It was found that the ratio of isophosphorylases is different for every mammalian species investigated and characteristic for the given species.