A study was conducted on the detection of ivermectin (22,23-dihydroavermectin B1) in cattle serum by thin-layer chromatography (TLC) after derivatization of this parasiticide by the known reaction with trifluoroacetic anhydride-1-methylimidazole and visual examination of the chromatograms under long-wavelength ultraviolet light. By derivatization of reference samples of ivermectin in acetonitrile, approximately 0.1 ng of a highly fluorescent material, tentatively identified as 2,5,6-tetradehydro-5,7-dideoxy-22,23-dihydroavermectin B1, could be detected on silica-gel thin layer plates. Extraction of fortified serum samples with methyl tert.-butyl ether followed by derivatization, hydrolysis, partitioning between water-hexane and TLC gave a limit of detection of 1-2 ng/ml. With these simple techniques ivermectin could be detected in cattle serum for 3-4 weeks after subcutaneous treatment of Hereford heifers.