The serologically active substance of the Brown-Pearce tumor, a complement-fixing antigen, differs notably from certain other tissue antigens (the Wassermann, Forssman, and organ- and species-specific tissue haptens, for example) in the fact that it is not effective after alcoholic extraction of the tissue containing it. Like many of the proteins and viruses it is in-activated upon heating to 65°C. for 30 minutes; and, like them as well, its activity is lost upon treatment with acid (to pH 4.5 or lower) or alkali (to pH 11.5 or higher).

Filtration and centrifugation experiments disclosed the fact that the antigen of the Brown-Pearce tumor passed readily through collodion membranes with average pore diameters of 383 mµ and more, but was retained completely by those with average pore diameters of 348 mµ and less. It was thrown down completely or almost completely upon centrifugation at 20,000 R.P.M. for one hour. The findings indicate that the Brown-Pearce tumor antigen has a large and nearly uniform particle size and weight,—as large as that of many of the viruses. They differentiate it sharply from the generality of "soluble antigens.rdquo;

Upon direct comparison, the complement-fixing antigen of the Brown-Pearce tumor was found to be similar in a number of its general traits to the serologically active substance of the virus-induced papillomas of rabbits, which in turn is intimately associated or identical with the virus responsible for the papillomas. Extracts of the Brown-Pearce tumor containing the serologically active substance in quantity have given rise to no lesions, however, upon injection into normal or tarred rabbits.

The significance of the findings is discussed.

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