In previous communications, this laboratory has reported on an antibacterial factor in both pure and whole saliva which is inhibitory to six species of lactobacilli and to three species of streptococci. Characterization of this factor indicates that the factor consists of two separable components by dialyzation, but neither one of which alone possesses any activity. When these components are recombined, the activity is restored and it is not identical with crystalline egg white lysozyme. Moreover, the factor is not absorbed by the homologous organisms, but is adsorbed on Bentonite. The factor was designated as the S.A. Factor. Assay organisms is found to be sensitive to the S.A. Factor in the logarithmic growth phase. The intensity of the Gram staining reaction decreases after treatment of bacteria with the S.A. Factor and metamorphosis do not occur.
The dialysable cofactor necessary for the activation of the S.A. Factor is identified as thiocyanate. Iodide in sufficient concentration would also substitute for the cofactor.
The non-dialysable portion of the S.A. Factor is successibly purified by the method of salting out, two steps of column chromatography, and sucrose density gradient centrifugation. The final material in the peak is revealed to be single substance by analytical ultracentrifugation.
The sedimentation constant (S₂₀, _w) is 5.82×10^-13, and has its maximum ultraviolet absorption at 279 mμ. Hexosamine content is 2 per cent of the total protein. No lytic action against Micrococcus lysodeikticus is observed with purified S.A. Factor by the method of Hartsell (1949).
The S.A. Factor is not detectable in the serum of human and some mammals. A substance which inhibits the S.A. Factor is present in the sera. The inhibitory substance was designated as the S.A.F. depressor. The depressor is non-dialyzable, heat stable, and not adsorbed on Bentonite. The purified serum albumin of human and bovine is identified as the depressor. The depressor activity of serum albumin is blocked by reagents of sulfhydryl group, such as PCMB, iodoacetate etc. It is suggested that sulfhydryl group of the albumin is essential to the depressor activity.
As a conclusion, the S.A. Factor is the firstly purified protein which shows the antibacterial activity in human saliva.

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