Using an in vitro system based upon radioisotope technique, the opsonizing and bactericidal effects uponE. coliof sera from germfree,E. coliandB. subtilismonocontaminated, and conventionalized rats, were investigated. Sera from the germfree animals were found to possess opsonizing as well as bacteriotoxic properties. The opsonizing activity was highest in sera from rats monocontaminated withE. coli, and lowest in sera from germfree animals. The bactericidal activity was lowest in sera from germfree rats. Specific and unspecific monocontaminations as well as conventionalization increased the serum bactericidal activity against the strain ofE. coliused.