Iseki and Sakai (1953, 1954) reported that antigenic conversion is possible from a strain of subgroup E1 of Salmonella group E, having somatic antigens 3, 10 to a strain of subgroup E2, having 3, 15, by means of phage 15, derived from bacteria of subgroup E2, and clarified that in bacteria of E2, prophage a 15 controls the production of somatic antigen 15. Later, Harada (1956) elucidated that phage E 34 from subgroup E3, which has somatic antigens (3), (15), 34, infecting bacteria of subgroup E2 can produce antigen 34 in them. Iseki and Kashiwagi (1955, 1957) further elucidated that bacteria of Salmonella groups A, B, and D that have somatic antigen 1 are lysogenic, and that phage iota, derived from these, when it infects bacteria without antigen 1, makes them lysogenic, at the same time producing antigen 1. That the production of antigen is controlled by temperate phage was also reported by Baron, Formal, and Washington (1957) concerning somatic antigen 20 of Salmonella group C, and by Matsui (1958) concerning type antigen IV of Shigella flexneri. These antigenic conversions can be induced not only by prophage but also by phage in vegetative phase. This was demonstrated with virulent phage obtained by mutation, by Stocker (1958a, b) concerning the production of somatic antigen 1 of Salmonella typhi murium, and by Uetake, Luria, and Burrous (1958) concerning 15 of Salmonella group E. Consequently this phenomenon is called "lysogenic conversion" (Lederberg, 1955) or "phage conversion" (Stocker, 1958a, b). In this paper it is reported how antigenic conversion is performed in Shigella Flexneri not only in the manner of 1a-4a and 3b-~ 4b, which were reported by Matsui (1958), but also in the manner of 1b-4b and 2a-4a. It is also made clear that the antigen which is related to the conversion in these cases is IV1, and that besides a phage from 4c (EW 19), one from 4b (EW 18) has also the same conversing ability. Materials and methods. 1. Strains. Strains of Shigella flexneri used in the present experiments are listed in Table I. 2. Examination for lysogenicity and sensitivity. Bacterial liquid, obtained by 8 hours culture at 37°C, was dried on an agar plate, and

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