Lactoperoxidase (LP), thiocyanate (SCN-), pH and somatic cell counts (SCC) were measured in mammary secretions from 20 cows collected 14 d before drying-off, 7 and 21 d after drying-off, and 3–18 d postcalving. The inhibitory activity of the secretions on Streptococcus uberis was determined and the susceptibility of the udder to infection by this organism was tested by intramammary infusion of 250 colony forming units at the above stages. LP, SCN-, pH and SCO increased during involution and fell postcalving. The secretions collected before drying-off, 7 d after drying-off and postcalving inhibited growth of Str. uberis.; those collected 21 d after drying-off did not. Inhibitory activity in pre-drying-off secretions was destroyed by heating and restored by addition of LP, glucose and glucose oxidase, but addition of these substances to secretion 21 d after drying-off did not provide a full inhibitory system. The growth of Str. uberis in the secretions was correlated with intramammary susceptibility, since challenges with Str. uberis at 14 d before drying-off, at 7 and 21 d after drying-off and postcalving led to 43·8, 25·0, 81·3 and 37·5% of quarters becoming infected. It is suggested that the LP/SCN-/H2O2 system plays a role in protecting the lactating mammary gland from infection with Str. uberis but becomes ineffective as involution progresses.