The preparation from several organs of normal mice and guinea pigs of an anti-Pasteurella pestis factor (APF), active in vitro, is described. The APF was active in varying degrees for several strains of P. pestis and for Micrococcus lysodeikticus but not for several other Gram positive or negative bacteria.

Crude and purified APF from mouse liver and crude APF from mouse and guinea pig spleen and kidney were active following storage at -20°C. On the other hand, the activity of both crude and purified guinea pig liver APF was lost during storage at -20°C, but could be restored by heating at 60 or 65°C but not higher. Evidence is presented for the nonidentity of APF with previously described antibacterial substances such as basic polypeptides, histone, spermine, phagocytin and lysozyme. The evidence consists chiefly of dissimilarity in method of preparation and of the differential action of extraneous materials on the activity.

One dose of APF administered at the time of inoculation with P. pestis did not protect mice from infection, and the virulence of P. pestis for mice was not altered by preliminary treatment of the organisms with APF in vitro.

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