The protective effect of bovine somatotropin (bST) during experimental Streptococcus uberis mastitis in cows was studied. The left quarters of 10 cows were infected with 500 cfu of S. uberis O140J. Five cows were subcutaneously treated with 500 mg of recombinant bST 7 d before and after infection, and 5 control cows received the excipient. In the treated cows, total milk production significantly increased after the first and second bST treatments. After infection, milk production decreased 24 and 40% in the infected quarters, 6 and 14% in the uninfected quarters, and 15 and 28% overall for treated and control cows, respectively. In the bST group, milk production was completely restored after 3 wk, but, in the control group, total production and the production of the infected quarters remained lower than preinfection production. The increase in somatic cell count occurred earlier and more rapidly in the control group, and the return to normal values was also more rapid in these cows. The amount of bacteria in milk was higher in the control cows. Changes in milk composition, such as lactose, protein, fat, Na+, K+, and Cl-, were significantly more pronounced in the control cows. Also, clinical symptoms were more prominent in the control cows. Somatotropin protected the mammary gland from excessive production losses and compositional changes during a subsequent episode of experimentally induced Streptococcus uberis mastitis and significantly improved the normalization of production and composition, which indicates a beneficial effect on the restoration of the integrity of the blood-milk barrier.