Escherichia coli strain RDEC-1 avidly adheres to rabbit ileal brush borders. Two separate experiments were designed to determine whether pili promote this adherence. (1) Adherence of strain RDEC-1 was phenotypically suppressed by changing the culture medium. Loss of adherence was correlated with the absence of pili. Thus, growth of strain RDEC-1 in Penassay broth (Difco Laboratories, Detroit, Mich.) promoted both adherence and expression of pili on greater than or equal to 90% of organisms, whereas growth in brain-heart infusion medium suppressed adherence and reduced the percentage of piliated organisms to less than or equal to 13%. (2) The adherence ability of strain RDEC-1 was genetically transferred to previously nonadherent and nonpiliated Shigella flexneri. The Shigella exconjugants that inherited the adherence ability were uniformly piliated, while all nonadherent Shigella exconjugants were nonpiliated. Finally, the pili on both RDEC-1 and the Shigella exconjugant strains were shown to be distinct from type 1 pili. Therefore, unique pili confer upon strain RDEC-1 the ability to adhere to rabbit intestinal brush borders.