A single injection into mice of the P-10 polysaccharide (polysaccharidelipid complex) from the 724 strain of Serratia marcescens led to the development of a high titer of agglutinating antibody and a pronounced tolerance toward the lethal action of the polysaccharide. The tolerance, which was manifested within 24 hours, reached its height on about the third day, when it was 100 per cent effective against a lethal dose of polysaccharide, declined sharply and was lost usually in 15 to 21 days. Agglutinating antibody was present only at low levels on the third day, but by the tenth day had reached a high titer which was maintained beyond 21 days.

When the polysaccharide was heated in neutral saline solution, there was a considerable loss in toxicity but little change in its ability to confer tolerance on mice. Heating did not diminish the antigenicity of the polysaccharide, as judged by tests of precipitinogen activity and of agglutinin production in mice.

Under the experimental conditions employed, the polysaccharide failed to induce anaphylaxis in guinea pigs.

Careful consideration of the experimental results in this paper and in earlier publications leads us to believe that the tolerance developed in mice by a single injection of polysaccharide is caused by the presence of protective antibodies; this tolerance, therefore, in all probability, has an immunological basis similar to that leading to the production of serologically demonstrable antibodies. We favor the interpretation that the preparation of polysaccharide contains two separate antigenic factors, one being a toxic factor, the other being responsible for the production of agglutinating antibodies.

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This research was supported in part by an American Cancer Society grant recommended by the Committee on Growth of the National Research Council, and was presented in part at the thirty-ninth annual meeting of the American Association for Cancer Research, Atlantic City, March 12–13, 1948.

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