Simian virus 40 mutants containing both a tsA mutation (rendering the 90,000 molecular weight [90K] T-antigen thermolabile) and a deletion between 0.54 and 0.59 map units (reducing the size and the amount of the 20K t-antigen) were used to transform Chinese hamster lung cells. The frequencies of transformation by the double mutants were comparable to that of the tsA mutant alone by both the focus and agar assays except when the cells were serum depleted before infection. Growth-arrested cells were transformed (using the agar assay) by the deletion mutants at less than 2% the frequency found when the 20K t-antigen was normal. Growth arrest had very little effect on the temperature sensitivity of the resultant transformed cell lines whether or not the deletion was present.