Growth and proliferation were studied of a transplantable mouse tumor, the adenocarcinoma EO 771 (Adca EO 771) growing in C 57 and in nude mice on the one hand, and of human tumors (renal cell and hypopharynx carcinoma) growing in nude mice on the other. There is almost no difference in tumor growth, histology and proliferation whether the Adca EO 771 grows in C 57 or in nude mice. However, there are great differences in this respect between the transplantable mouse tumor and human tumors. Growth of the Adca EO 771 and C 57 and in nude mice occurs according to the Gompertz function, whereas growth of human tumors in nude mice differs, some tumors grow exponentially and some according to the Gompertz function. The proportion of necrotic tissue strongly increases with increasing tumor size in the case of the Adca EO 771, while it is about constant in human tumors regardless of the tumor size. The tumor cell density of the Adca EO 771 increases considerably with increasing tumor size, however, it remains about constant in human tumors. Concerning tumor cell proliferation an S phase duration was found that is rather similar for the cells of the transplantable mouse tumor as well as of the human tumors suggesting that DNA synthesis might be regulated by the host organism. A quantitative study of the growth of the metastases of the Adca EO 771 exhibited an allometric correlation between the growth of the metastases and that of the primary tumor. This leads to the consequence that metastases might originate later than estimated until now assuming exponential growth of metastases. Treatment of the Adca EO 771 with cyclophosphamide results in the death of almost all tumor cells; however the tumor repopulates. The toxic effect of cyclophosphamide on the mouse organism strongly depends on the size of the tumor at the time of treatment.