An interlaboratory study of 3 methods for the determination of sulfamethazine in swine muscle and liver tissue was conducted. A gas chromatographic (GC), a combined gas chromatographic-mass spectrometric (GC/MS), and a colorimetric method were evaluated. Four fortified and 2 incurred levels of sulfamethazine ranging from 0.00 to 0.45 ppm were studied. The study revealed that the GC/MS procedure provides the most reliable data and that the GC method provides acceptable data w i t h the incorporation of appropriate controls. The recoveries for the GC/MS method typically ranged from 90 to 100% for swine liver and muscle. The GC and GC/MS methods have been adopted official first action.