The acetylator status of 40 children with pulmonary tuberculosis was investigated by (1) sulfadimidine (SDM; sulfamethazine) acetylation test in urine and (2) estimation of isonicotinic acid hydrazide (INH) levels. The antimode was at 70% based on the frequency distribution of SDM acetylation. Children acetylating less than 70% of administered SDM were taken as slow acetylators while those with more than 70% as rapid acetylators. The serum INH antimode was at 0.85 micrograms/ml. Thus serum values less than 0.85 micrograms/ml categorised a child as rapid and those with more than 0.85 micrograms/ml as slow acetylators. The sensitivity of these two methods was similar with a correlation coefficient r = 0.64. Thus the determination of the type of acetylator by SDM acetylation test is equally reliable and technically simpler and is recommended instead of INH serum concentration.