An open, block randomised multi-centre clinical trial was performed in Norway during 1985 to 1987 to compare the therapeutic efficacy of three antibiotic regimens against clinical bovine mastitis caused by penicillin-sensitive bacteria. Two regimens consisted of procaine penicillin injected intramuscularly for either three or five days, and the third, the traditional Norwegian regimen, consisted of one intramuscular injection of a combination of procaine penicillin and dihydrostreptomycin followed by one intramammary treatment daily per infected quarter for four days. The study included 621 quarters with infectious mastitis from 439 cows. The most efficient regimen for all bacteria was five days systemic treatment (53.1 per cent cured), and the traditional regimen was second best (46.7 per cent cured). The least efficient regimen consisted of systemic therapy with procaine penicillin for three days (36.9 per cent cured). The difference between the therapeutic efficacies of the three regimens was reduced when the clinical mastitis was severe, and in severe mastitis caused by Staphylococcus aureus the difference was very small.