Milk from dairy cows never known to have had an intramammary infection with Streptococcus uberis can inhibit growth of Str. uberis for up to 7 h. This inhibition is abolished if milk is heated to 80 °C. Inhibition appears not to be related to immune defences as it occurs in skimmed milk (cell free), is unrelated to the concentration of immunoglobulin and survives heating to 56 °C. The effect is partly overcome by addition of selected amino acids and vitamins. It is suggested that the inhibition is caused by a restriction in the supply of essential nutrients part of which may require the conversion of plasminogen to plasmin.