An examination of teat canal swabs established that 51 teat canals out of 68 quarters of machine milked cows were colonized by Staphylococcus aureus. Only 31 of these quarters yielded milk from which S. aureus could be cultured, and 6 out of the 31 produced milk containing somatic cell counts in excess of 500 000/mℓ. No inhibitory substances could be detected in milk samples 12h after 10 mg of sodium cloxacillin had been deposited in the teat canal on 1-4 successive occasions. Teat canal swabs and milk sample cultures of the same quarters became and remained bacteriologically negative for at least a week after the last treatment. Six quarters, which according to the International Dairy Federation criteria were suffering from subclinical mastitis, became negative after local teat canal therapy. Scanning electron micrographs of one infected teat canal revealed the presence of cocci in depressions and crevices on the epithelial surface, suggesting that such cocci are not always flushed out into milk samples. Teat canal therapy should make a marked contribution to the control of bovine mastitis.