Two trials of oral neomycin therapy resulted in the reduction of the urinary excretion of methylmalonic acid and a fall in the plasma level of this metabolite in a child suffering from methylmalonic aciduria. It is quite probable that the antibiotic effect on the gastrointestinal bacteria resulted in decreased propionate production; propionate is the immediate precursor of methylmalonic acid. This therapy might be of special benefit during periods of acute acidosis in this disease.