Two strains of Streptococcus uberis, one (0140J) resistant to killing by purified bovine polymorphonuclear leucocytes suspended in milk and the other (EF20) readily killed by polymorphonuclear leucocytes were each infused into a mammary quarter of 18 lactating and 10 pregnant non-lactating cows. In the lactating cows 0140J produced clinical disease in 16 of 18 quarters whereas EF20 produced clinical disease in only two of 18 quarters. With the exception of three cows exposed to EF20, the quarters which resisted infection did so without apparent inflammatory reaction. In non-lactating cows both organisms produced clinical disease in six of 10 quarters. Two cows apart, a non-lactating udder was either resistant or sensitive to both organisms.