Experimental design was applied to an agar-diffusion inhibition test for the screening of residues of veterinary antimicrobial drugs in slaughter animals. The effects and interactions of four independent parameters were studied: the dextrose content of the culture medium, the trimethoprim concentration in the culture medium, the thickness of the agar-layer and the preincubation time. The effects on the inhibition zones of sulphadimidine, oxytetracycline, streptomycin and tylosin, substances of four different antibiotic groups were measured. An optimized multi-residue test was established with the parameter values that yielded the best sensitivity for the four substances