The frequency of colony formation in monolayers of cultured frog cell lines treated with puromycin was compared in (a) haploid and heteroploid lines and (b) mutagen-treated and nontreated haploid lines. Evidence that resistant colonies result from gene mutation was negative, since the colony frequency is independent of both ploidy and mutagen treatment. A study of five frog cell lines showed that colony formation in puromycin depends on (a) the concentration of puromycin, (b) preselection of the population with puromycin, and, particularly, (c) the capacity of the treated population to survive some exposure to puromycin. One haploid and one heteroploid strain showing stable resistance to puromycin have been isolated; comparison of those variants with sensitive populations has shown that resistance to puromycin is correlated with the cells' capacity to exclude the drug. The evidence for different levels of membrane permeability, combined with evidence for many degrees of resistance among and within cell populations, suggests a model of self-determining membrane units. The evolution of a resistant phenotype may result from changes in the proportion of specific units in the membrane population.

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