The presence of pathogenic properties was studied in 50 strains of S. flexneri 5 E. coli cultures and 267 sexual recombination by infection of the guinea pig conjunctiva. As noted, 20 strains of S. flexneri and 39 hybrids induced specific keratoconjunctivitis. Although S. flexneri hybrids differed considerably by their biological properties from the initial cultures, they retained their pathogenic properties. E. coli strains possessed no such properties.