The records of milk yield and infection with Streptococcus agalactiae in four herds over periods of 9–13 years were analysed, using rigorous methods of control in the comparison of yields of infected and healthy cows. Fifty-seven cows which became infected in the second to sixth lactations yielded significantly more (56·5 ± 20·4 gal./lactation; P = 0·01) before infection than did non-infected animals in corresponding periods. In thirty-two animals infected in the second to fifth lactations an average apparent loss of 194·2 ± 28·3 gal. was found for the first lactation in which infection was detected. This represents a loss of about 24 % of the lactation yield.