Multicell spheroids may prove useful in evaluating the interactions of mutagens with cells exposed in a tissue-like environment. However, direct comparisons among populations of Chinese hamster V79 spheroids of different sizes or with monolayers are complicated by the observation that as spheroids enlarge, the fraction of mutant cells resistant to 6-thioguanine (TGr) gradually decreases from about 5 in 10(5) to less than 1 in 10(5). There appear to be at least 2 explanations for these observations. First, TGr cells grow less well as spheroids than do 6-thioguanine-sensitive (TGS) cells. Second, the clonal nature of spheroid growth means that small samples of spheroids are likely to contain fewer pre-existing TGr cells.