In some pig herds it is common for 20% or more of pigs born alive to die before weaning, indeed the national average across all herds is approximately 12% (Meat and Livestock Commission, 1983). A large number of these deaths are caused by Esherichia coli, of which up to half are caused by the K88 serotype (Walters and Sellwood, 1982). Several reports (eg. Gibbons, Sellwood, Burrows and Hunter, 1977) have shown that resistance to K88 E. coli is inherited as a simple Mendelian recessive. Of potential practical significance is the evidence that suggests that the sole use of resistant boars within a herd would prevent any K88 scour since the only susceptible progeny produced would be born to susceptible dams which would provide passive maternal protection to their piglets. However, the obvious advantage in using resistant boars in a herd would be nullified if performance of the offspring was significantly lower in important economic traits. Two trials on different farms were set up to investigate this possibility.