The present evidence indicates that Shigella-like pathogenicity is determined by a multiplacity of genes. Although deliberate attempts have been made to confer invasive virulence on E. coli strain K12 by employing classical procedures of recombination with virulent S. flexneri donor strains, they have not yet been successful. While we should, theoretically, be able to achieve this, the practical problem of testing for pathogenicity precludes screening larger numbers of hybrid clones for the acquisition of virulence. This increases the difficulty of successfully realizing that end. Nevertheless, since invasive-type pathogenicity is determined by multiple genetic loci, we consider it unlikely that random insertion of foreign DNA into the E. coli K12 genome could supply all of the genetic information necessary to convert this organism into an invasive enteric pathogen.