The four-plate test (FPT) method was used for the determination of sulfadiazine/trimethoprim residues in trout in the European Community (EC). This microbiological inhibition test used three media seeded with Bacillus subtilis at different pH values (6, 7.2 or 8.0) and a fourth medium seeded with Micrococcus luteus. The Premi test was also used for the determination of sulfadiazine/trimethoprim residues in trout. The effects of a trout muscle supernatant on detection limits of antibiotics with microbiological inhibition tests were measured. The supernatant was applied directly on top of the paper disks impregnated with aqueous antibiotic solutions. Inhibition zones were compared with those obtained by the same standard solution of sulfadiazine/trimethoprim without the trout supernatant. The detection limit of penicillin, sulfadimidine, streptomycin groups of antibiotics were determined for checking the test plates. For supernatant from fish fed with pellets containing antibiotics (sulfadiazine/trimethoprim), the inhibition zones increased according to the duration of the feed application. In the control group, no inhibition zones were detected. The concentration of the residues accumulated and reached a plateau after 5 days. The antibiotics were detectable in the same concentration as on day 7, but three days later (on day 10) they were no longer detectable in the fish samples. In contrast to the 'FPT', 3 days after discontinuing the medicated diet, there were still residues detectable by 'Premi test' on day 10. Data is presented that shows that the FPT and the Premi test methods are very useful for the determination of sulfadiazine/trimethoprim residues in trout.