1. Through a set of preliminary injections of colloidal copper or hirudin, given from the second to the sixth day after transplantation, we can weaken the effect of injections of the same substances given from the ninth to the thirteenth day after transplantation.
2. Injections of colloidal copper given before the transplantation of the tumor lead to a weakening of the effect of injections given from the ninth to the thirteenth day after transplantation.
3. The same result can be obtained if we inject colloidal copper or hirudin into mice from the second to the sixth and from the ninth to the thirteenth day after transplantation, and use these tumors at the end of the series of injections for further transplantation into another set of mice; the developing tumors are more resistant to the action of colloidal copper or hirudin than new control tumors.
4. We may therefore conclude that the immunity has two sources: (a) it is based on changes taking place somewhere in the host organism; and (b) it is localized in the tumor cells themselves which transfer this immunity to the following generations of tumor cells.
5. The immunity acquired against colloidal copper does not protect noticeably against the action of hirudin, and vice versa. The immunity is therefore specific.
6. We discuss certain general conclusions which may be drawn from these experiments. We expect to test the validity of these results in further experiments.