The effectiveness of on-farm tests for the detection of antibiotic residues in milk and urine was examined using the Virginia Tech dairy herd. Milk samples from treated cows were collected 72 hours post treatment and tested for residues by the Bacillus stearothermophilus Difco disc assay, the Delvotest-P, and the Penzyme test. Urine of cull cows was collected and tested with the Live Animal Swap Test (LAST). Results showed 21% to 24% of milk samples contained antibiotic residues past the recommended withholding period. In a comparison of the two on-farm tests to the officially recognized B. stearothermophilus disc assay, no significant difference was seen between the tests. An inordinately high (62.5%) level of urine samples from cull cows exhibited antibiotic combination with the LAST, although all animals had completed the recommended withholding period specified for each antibiotic. In the second phase of this study, cows in the Virginia Tech dairy herd were assigned to experimental or control groups. Milk from cows in either group whose linear somatic cell count score (SCC) ≥ 5 for the first time during that lactation was culture on blood agar plates to detect mastitis pathogens, and the SCC was determined. Experimental group cows were treated in those quarters with a SCC ≥ 5 while control cows had antibiotic therapy administered based only on clinical symptoms. Treatment group had no significant effect on milk production or SCC, however treatment of elevated SCC quarters did result in trends for decreased SCC and reduced milk yield loss. Treatment group significantly altered infection status of the cow. Of the quarters cured of infection, a greater percentage (63%) belonged to the experimental group. Results from this study indicate that the practice of administering antibiotic treatment based on elevated SCC should not be adopted until further studies on herds of higher infection rates are conducted. / M.S.