Relatively simple and inexpensive procedures for screening milk for sulfamethazine (SMZ) and one of its metabolites, N4-acetylsulfamethazine (ASMZ), are detailed. Both methods detect at the low parts-per-billion level and are suitable for both field and laboratory use. Milk is passed over Chromosorb 102, which adsorbs SMZ. The drug is eluted and purified by direct passage of the effluent over small beds of buffered anion-exchange resins and alumina and is finally isolated and detected colorimetrically. For ASMZ, the procedure is modified so that SMZ is removed in the purification steps. The isolated ASMZ is then hydrolyzed to SMZ for detection. Application of the methods 5 years apart (1988 and 1993) shows that SMZ is still being used but to a lesser extent in 1993. Of over 250 samples screened in the 2 studies, only 2 were estimated to contain SMZ at 10 ppb, and the majority contained SMZ at 1 ppb. ASMZ was detected in a number of samples that were negative for SMZ.