AbstractGrowth rates of spontaneous tumors of C3H and C3HB inbred mice were averaged together. Four series of tests using adenoisine as an inhibitor of tumor growth were used in a 0.05 molar concentration. The four series used were, (a) adenosine I, (b) adenosine II both for intraperitoneal injections, used singly and then combined for comparative analysis, (c) adenosine III, and (d) adenosine IV, both used for subcutaneous injections, of which series III was used in distilled water and series IV in physiological salt solution. Observations on tumor size were plotted at five time‐period intervals in order to determine growth rates. It was found that adenosine IV, in physiological salt solution, gave the greatest deviation from from the controls both in inhibiting tumor growth and also in stimulating tumor growth in mice of the different age and generation classes as determined in a lineal descent from a mouse which had received a liver‐emulsion injection. The subcutaneous injections of adenosine III gave only an early inhibition of tumors whereas the intraperitoneal injections of adenosine I and II brought about a constant inhibition of tumor growth during the life span of tumor‐bearing mice. Variations in the effect of adenosine upon tumor growth were found to be dependent upon 1. the age of the mouse and 2. the generation of descent removed from the original liver treatment (Strong, 1969a). These tests for tumor growth, therefore, provided a close examination of some of the variables involved in the use of adenosine as an inhibitor and as a stimulator of spontaneous tumor growth rates in