We have investigated the qualities of one R factor 552 discovered on a strain of S. typhi resistant to A, C, S, T, nontypable, isolated from stool cultures; from the same patient, before starting the treatment we isolated, from his blood sample, the strain S. typhi 221, sensitive to A, C, T, degraded phage-type Vi A. Factor R 552 fi- when infecting strains of S. typhi Vi A and of A degraded 221- leads to the conversion of the respective phage-types into non-typable ones, as a result of the restricting and modifying effect on phage Vi A and on the derivatives resulting from it. Derivative R 552-1 as a resistance marker to ampicilline has a restrictive effect on the phage of S. panama A 47 too. Not taking into account possible causes such as spontaneous mutation, lysogeny, and adsorption of phages, we reach for the conclusion that R factor 552, through is restrictive effect, is the only cause responsible for the existence in the same patient of two strains of S. typhi different from the point of view of phage-type and antibiotype.