This study was carried out on five types of experimental tumors maintained by serial subcutaneous transplants in isogeneic mouse hosts. These tumors involved three mammary carcinomas (dbrB, DBAH, MT2), a spindle-cell sarcoma (TEC) and a lymphoblastic type of lymphoma (DBA/3). Growth curves of these tumors are presented. Computed percent labeled mitoses curves for the five types of tumors, the derived cell cycle parameters (TG1, TG2, TS, Tc), and the volume-doubling time (VDT) in days are also presented. The histologic and morphologic appearance of each type of tumor is seen by light microscopy, and the ultrastructural morphology of each type of tumor is seen in electron micrographs. The variation in the kinetic parameters and the autoradiographic exposure time needed to obtain comparable labeling intensity for the five types of tumors is interpreted on the basis of the ultrastructural integrity of the cytoplasmic components of the individual tumor type. The response of these five tumor types to radiotherapy was investigated. The therapy consisted of administering combined treatments of three agents: X-rays, the radiosensitizing drug, misonidazole, and microwave hyperthermia. This treatment resulted in an enhancement factor of 3.9, compared with that of X-rays alone. Total tumor regressions were obtained with microwave hyperthermia alone. The required time of exposure to hyperthermic treatments differed, depending on the response of each type of tumor.