An analytical system was developed for detection of antibiotic residues in bovine milk. The method is based on competitive fluorescent immunoassays in glass capillary tubes (U.S. Patent No. 5,624,850). The system consists of an assay cartridge containing 4 glass capillaries, a reagent tray with 4 wells of dried reagents, and a Parallux processor, which processes the assay, reads fluorescent output, and reports test results. Minimum sensitivity for detection of 6 beta-lactam antibiotics in bovine milk was determined to be penicillin-G, 3.2 ppb; ampicillin, 2.9 ppb; amoxicillin, 3.6 ppb; cloxacillin, 7.4 ppb; cephapirin, 16.3 ppb; and ceftiofur, 33.7 ppb. The assay system was also specific and sensitive for detection of incurred residues at U.S. Food and Drug Administration tolerance levels: penicillin-G, 5 ppb; ampicillin, 10 ppb; amoxicillin, 10 ppb; cloxacillin, 10 ppb; cephapirin, 20 ppb; and ceftiofur, 50 ppb. There was no interference in detection of minimum sensitivity levels of antibiotic by the presence of somatic cells at approximately 1 x 10(6) cells/mL. Milk containing 3 x 10(6) cells/mL bacteria commonly found in mastitic milk also showed no interference when tolerance levels of antibiotic were present. There was no detectable interference on results by a wide variety of non-beta-lactam drugs.