The activity of plasma creatine kinase activity was investigated in Swiss albino mice, in mice with hereditary muscular dystrophy (strain 129) and their heterozygote littermates. In affected mice enzyme activity was markedly elevated (2 to 10 times the normal mean). No difference was noted between the enzyme activity of mice aged 6 and 10 weeks. Heterozygotes had the same plasma enzyme activities as Swiss albino mice. It is suggested that hereditary dystrophy in mice is more similar to the limb girdle type of human muscular dystrophy in respect to plasma enzyme changes. This would be in agreement with the cause of the disease, which in both instances is an autosomal recessive gene.