1. The histological and physiological characteristics in vitro of mouse tumors have been compared.

2. Twenty-eight adenomas, 31 adenocarcinomas, 18 carcinomas, and a series of tumors from 2 transplantable carcinomas and from Ehrlich carcinoma have been cultivated in vitro.

3. The adenomas were characterized by cells of normal appearance, migrating quickly and organizing in thin membranes, by few mitoses, a large acid production, and a rapid digestion of fibrin and egg albumin.

4. The carcinomas differed from adenomas in a less rapid and extensive migration of epithelial cells, a tendency to grow in bud-like formations, the presence of many mitoses, and a lesser acid production and fibrin and albumin digestion.

5. Ehrlich carcinoma differed from both adenomas and spontaneous carcinomas in the unhealthy state of the cells, their irregular growth, and the occurrence of a great many mitoses. Acid production was slight, and digestion of egg albumin or fibrin less marked, and even entirely lacking.

6. The adenomas and spontaneous carcinomas did not invade embryonic tissue. The transplantable carcinomas invaded it slightly, while Ehrlich carcinoma actively infiltrated and destroyed it.

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