An assay method for the methylmalonyl-CoA mutase of leukocytes obtained from 3 ml of blood was established. The enzyme activity which was measured with or without the in vitro addition of 5'-deoxyadenosylcobalamin was found to be of value for the diagnosis of two variants of methylmalonic acidemia (vitamin B12 responsive and unresponsive), and also for the detection of heterozygotes with the vitamin B12 unresponsive type.