Experimental infections of the mammary gland of newly calved cows with 500 serum resistant Escherichia coli produced a very severe form of mastitis when compared with animals in mid-lactation. Ten hours after infection the bacteria had multiplied in the milk to very high numbers (10(6)--10(7)/ml) and the animals showed signs of pyrexia, anorexia and diarrhoea. Initially the gland and milk showed little or no clinical signs of mastitis, but later the secretion became a viscous, serous fluid with little or no casein or fat. A delay in diapedesis of neutrophils into the gland appears to be the reason for the peracute state and lack of clinical signs. This form of pathogenesis may produce a paradoxical situation where the most severe cases of E coli mastitis cannot be diagnosed at a stage early enough for the animal to respond to therapy.