Growth curves for multicellular spheroids of 15 different tumor and normal cell lines were characterized by doubling times which decreased with increasing growth until a stable saturation was attained. In spite of the identical and constant conditions during growth, the size at saturation varied by factors of 67 in spheroid volume and 75 in cell content. These saturation sizes showed no correlation with the monolayer doubling times or clonogenic efficiencies, the initial spheroid growth rate or clonogenic capacity at saturation, the cell packing density, or the species of origin and type of cell line. There was a strong correlation between the maximal spheroid size and the size at which necrosis initially developed, suggesting control by necrosis. Crude extracts prepared from spheroids with extensive necrosis showed dose-dependent cytostatic and cytotoxic activities against monolayer cultures, while similar extracts from spheroids without necrosis had little effect. This activity was also detected in the culture medium to which the large spheroids had been exposed prior to preparation of extracts, suggesting that the responsible factor(s) can diffuse through the spheroid. The extract from spheroids of one cell line inhibited the growth and clonogenicity of four other cell lines, including human diploid fibroblasts. DNA content profiles measured during exposure to this extract showed that the cytostatic effect was not due to the arrest of cells in a specific cell cycle phase. The cell volumes were increased during culture in medium containing the extract from spheroids with extensive necrosis. These data support the hypothesis that growth saturation in spheroids is regulated by factors produced, released, or activated during the process of necrosis and suggest that these toxic factors have potential therapeutic use.