Ethambutol adsorption was rapid and serum concentrations were proportional to the dose administered. A dose of 50 mg/kg of body weight produced a peak serum concentration of 10 gamma/ml at two hrs. Daily peak concentrations of 5 gamma/ml (previously shown to be highly effective against experimental tuberculosis in the mouse and monkey) were produced by single daily doses of 25 mg/kg. The serum concentration at 24 hrs, after dosage was about 10% of the peak concentration. And approximately 1/2 of the dose was excreted in the urine. No significant drug accumulation was observed in the serum .