Two strains of Streptococcus uberis (0140J and EF20), previously shown to differ markedly in their ability to infect the mammary gland and cause clinical mastitis, showed correlated differences in their resistance to some host defence mechanisms. Both strains produced a hyaluronic acid capsule but strain 0140J, the more infective, was more resistant than EF20 to phagocytosis and killing by bovine neutrophils. Loss of the capsule from strain EF20 during the stationary phase of growth did not affect its resistance to phagocytosis. Strain 0140J also grew more rapidly than EF20 in raw skimmed milk but dithiothreitol substantially reversed this inhibition, suggesting the involvement of the lactoperoxidase system in the inhibitory effect of raw milk.