Summary1) Acetylation of sulfanilamide and of isoniazid in pigeon liver extracts was markedly inhibited (non-competitively) by 4-amino analogues of folic acid. A-methopterin (105 M) and aminopterin (5 × 105 M) inhibited acetylation by 62 and 58%, respectively. Folic acid (10-3 M) was not inhibitory, and failed to reverse the inhibition by A-methopterin. 2,4-Diamino-6,7-diphenylpteridine (10-3 M) gave 50% inhibition of acetylation and 2,4-diaminopteridine (10-3 M) was inactive. A-methopterin, administered to rabbits conjointly with sulfanilamide, produced a marked increase in plasma level of free sulfanilamide, and a concomitant decrease in acetyl-sulfanilamide. 2) The possible significance of these results with regard to combination chemotherapy of cancer was discussed.