Multicellular tumor spheroids (MTS) from 7 mouse solid tumors [fibrosarcoma, alveolar cell carcinoma, mammary carcinoma] were compared in terms of their growth rate and the fraction of cells in cycle. Growth rates ranged from a low of 19 .mu.m/day through a maximum of 106 .mu.m/day, demonstrating a wider range of growth rates when the same tumors are grown as monolayers. Through the use of an [125I]iodouridine deoxyribose incorporation assay, it was possible to demonstrate that cells within the MTS that are in cycle double at a slower rate than in monolayers and the depth of the dividing shell ranges from less than 20 to more than 100 .mu.m. The depth of the dividing shell or the growth fraction is highly correlated with the MTS growth rate (r = 0.97). The major factor that determines the wide range of MTS growth rates, in spite of relatively uniform doubling times in monolayer, is the fraction of cells in cycle.