We have described the methods used in determining the influence of certain substances on tumor growth, and we measured approximately the degree of reliability of the quantitative method used. We examined with these methods various classes of substances,— distilled water, a number of inorganic salts, inorganic colloidal substances, various organic colloidal and non-colloidal substances, especially various proteids, tuberculin and hirudin alone as well as in combination with other substances. Distilled water, various inorganic sulphur preparations, and various inorganic salts did not show an inhibiting effect on tumor growth sufficient to be detected by means of our first method. Only in the case of gold potassium cyanide was there possibly a slightly retarding influence present. On the other hand, certain colloidal solutions of heavy metals (copper, platinum, gold) retard the growth of a number of tumors of injected animals. Certain combinations of copper salts and casein act in a similar manner. Of the organic substances used, casein, nucleoproteid, and hirudin were active, while the other proteids tested, as well as various other organic substances and tuberculin and lecithin, seemed to be either without effect or weaker than the other substances mentioned as retarding the tumor growth. Hirudin was active and caused in addition to its inhibiting influence the retrogression of a certain number of tumors. Especially active was a combination of hirudin with colloidal copper and of hirudin with nucleoproteid.

One single injection of casein or nucleoproteid, or of the Heyden preparation of colloidal copper, leads to a more or less marked edematous condition of a certain number of tumors, while hirudin caused in addition, in many cases, marked hemorrhages in or around the tumors. Other substances which we tested did not show this effect, although their inhibiting action on tumor growth may have been equally strong. Very young tumors (two to six days old) are not retarded in their growth through injection of colloidal copper or hirudin, while nine to thirteen days old tumors are, independently of their size on the ninth day, inhibited in approximately the same relative degree; absolutely, however, the more rapidly growing smaller tumors are more markedly inhibited than the normally more slowly growing larger tumors.

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