The parental HeLa cell population, a morphologically uniform, human cancer cell strain, grown for several years in tissue culture by procedures always involving massive inocula, has been shown to contain different mutant cell types.

Two clonal lines have been isolated and established as reliable stock cultures. Both strains exhibit 100 per cent plating efficiency in high or low serum concentrations in the presence of a feeder system. In the absence of a feeder system and in low serum concentrations, the two strains are quantitatively differentiable: S3 still exhibits 100 per cent plating efficiency, while that of S1 lies in the neighborhood of zero.

These differences have remained stable throughout 100 successive generations of growth of each strain including 2 single cell isolations.

Application of these techniques to studies in the genetics of mammalian somatic cells and to specific cell-cell interactions has been indicated.

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