Decimal dilutions containing 5 X 10(5) to 5 X 10(1) cells were inoculated s.c. into nude mice and the course of tumor development was recorded. The highest concentration of cells produced rapidly growing poorly differentiated sarcomas within 2-3 weeks of their inoculation. Upon explantation the resultant tumors yielded cells which multiplied on plastic almost as rapidly as did their progenitors used to initiate the tumors, and had as high a colony forming efficiency in agar as long as tryptose phosphate broth was omitted from the agar medium. Tumors initiated by the lower concentrations of cells were disproportionately delayed in their appearance and tended to increase in size at a low rate. At least one tumor regressed and one which apparently regressed appeared again at a later time. These changes are characteristically described under the rubric of tumor regression. Host reactive cells such as neutrophils, eosinophils, macrophages, and fibroblasts were observed in some tumors. One of the tumors was a low grade hemangiosarcoma, another a well-differentiated fibrosarcoma, and the rest poorly differentiated sarcomas. Cells from two tumors initiated by 500 and 5000 cells multiplied slowly in early passages in culture, particularly when seeded at low densities at which they appeared to sustain cumulative damage even when multiplying. In later passages, the "low dose" tumor cells gained the capacity to multiply in culture after seeding at low densities, but it took up to 50 cell generations to reach this capacity. The loss of growth capacity on plastic of cells from the low dose tumors and its subsequent restoration by passaging in culture may provide a quantitative method for analyzing the type of cellular change which underlies tumor progression.