Two lactational intramammary antibiotic preparations (cloxacillin and oxytetracycline) were each used to treat eight goatsby intramammary infusion in one half. The rate of release of the antibiotics was monitored by a diffusion assay based on the sensitive organismBacillus stearothermophilusvar.calidolactis. Cloxacillin was still detectable 156 hours afterthe final treatment while oxytetracycline was undetectable 108 hours following treatment. A small but significant amount of antibiotic was translocated between treated and untreated halves of the udder in both cases.