An in-vitro test that demonstrates adhesion of K88-positive Escherichia coli to brush borders prepared from the small intestine of the pig is described. K88-positive E. coli adhered to the brush borders from some pigs ("positive" pigs) but not others ("negative" pigs). The sires of the pigs tested could be placed into two groups, namely, those that sired only "positive" piglets, and those that sired a mixture of "positive" and "negative" piglets. The incidence of the two phenotypes in litters indicated that "positive" and "negative" piglets arose as a result of simple Mendelian inheritance. It is suggested that "negative" pigs could be bred and that they might have a natural resistance to neonatal infection with K88-positive E. coli.