A case of methylmalonic aciduria is described. The clinical course was unusually mild, the child surviving to the age of 8 years. Studies on cultured fibroblasts confirmed a defect in propionate metabolism which was non-responsive to hydroxycobalamin in vitro. Polyethylene-glycol-induced cell fusion with a known methylmalonyl co-enzyme apomutase-deficient cell line showed genetic complementation indicating that in this patient the defect was in one of the enzymes required for 5-deoxyadenosyl cobalamin synthesis.