The effectiveness of various assay systems for the examination of trout was investigated. The Four Plate test (FPT), Premi test and ELISA test were used for determining sulfadiazine/trimethoprim residues in trout. According to the results of the Four Plate test, when trouts were fed with pellets containing antibiotics (sulfadiazine/trimethoprim), the inhibition zones increased according to duration of 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 10, but 5 days later (on day 15) they were no longer detectable in the trout by EEC FPT. In contrast to the FPT, 5 days after discontinuing the medicated diet, there were still residues detectable by the Premi test and ELISA test on day 15. According to the ELISA test results of the trout; from day 1 to day 10 the results of sulfadiazine residues were found to be 4.18, 8.02, 9.13, 20.10, 21.32, 20.52, 20.75, 20.99, 20.90, 20.32 ppb, respectively. On day 10, the antibiotics were detectable in the same concentration. On day 15, sulfadiazine residues were found to be 6.82 ppb, but 5 days later (on day 20) they were no longer detectable in the trout. EEC Four Plate method presented less sensitivity and effectiveness in the detection of sulfadiazine.