Yersinia enterocolitica is an enteric pathogen that has served as a model system for the study of microbial pathogenesis. Numerous virulence gene have been identified both on the virulence plasmid and on the chromosome. One of the chromosomal genes that is highly correlated with virulence is ail, a gene identified along with inv in a screen for Y. enterocolitica genes that could confer an invasive phenotype to Escherichia coli. Ail also promotes serum resistance in both E. coli and Y. enterocolitica. Several virulence factors homologous to Ail have been identified in other pathogens, yet very little is known about what constitutes the functional domain(s) of these proteins. Proteins in this family are predicted to consist of eight transmembrane beta-sheets and four cell surface-exposed loops. We constructed and characterized a number of insertion, deletion and point mutations in the regions of ail predicted to encode the cell surface loops. The results from the analysis of these mutants indicate that cell surface loops one and four do not directly promote invasion or serum resistance, whereas mutations in loop three appear to modulate both phenotypes. Analysis of mutations in loop 2 suggests that this surface-exposed loop contains sequences required for serum resistance and invasion. In addition, a peptide derived from the sequence of loop 2 was able specifically to inhibit Ail-mediated invasion in a dose-dependent manner. These results suggest that Ail directly promotes invasion and that loop 2 contains an active site, perhaps a receptor-binding domain. Analyses of the mutations also suggest that the serum resistance and invasion phenotypes may be separable, because there are numerous mutations that affect one phenotype but not the other.