A study on the kinetics of excretion in milk of some antimastitis drugs is described. The five antibiotic molecules used were detected by confronting two methods: the Galesloot-Hassing method which, even though specific for the detection of penicillin in milk, is officially used for the detection of antibiotic residues in general; and the M. luteus paper disk assay, which uses M. luteus ATCC 9341 as test microorganism. The obtained results show that withdrawal times for the assayed molecules are longer than those declared and that M. luteus is more efficient for the detection of kanamycin residues.