TFTr mutants of L5178Y/TK+/- mouse lymphoma cells are analyzed as they appear in situ following cloning and incubation for 9-11 days in soft agar cloning medium. These TFTr mutants can be divided by colony size into sigma, small colony, and lambda, large colony, mutants. The use of a size discriminator on an automatic colony counter allows the production of histograms to evaluate the size distribution of colonies on a plate. The evaluation of these size distribution curves provides insight into the properties of sigma and lambda mutants. From these analyses several conclusions may be drawn. The sigma phenotype is preferentially associated with the TFTr subpopulation of a treated culture. The sigma phenotype is not an artifact of delayed toxicity following treatment. The frequency of quantifiable sigma mutants is not affected by agar concentrations between 0.20% and 0.45% in the cloning medium. TFTr sigma mutants are produced spontaneously and can be induced by a variety of mutagens. The decline in overall detectable mutant frequency observed for some mutagens with increasing time after treatment is due to the decline in sigma mutant frequency. The quantitation of both sigma and lambda mutants is thus useful in obtaining maximum utility of the information provided by the L5178Y/TK +/- mouse lymphhoma assay.