Chromosomal constitution and amethopterin resistance in cultured mouse cells.

The chromosomal constitutions of four amethopterin-resistant sublines of cultured Sarcoma 180 cells were compared with those of two sensitive sublines. No correlation between amethopterin resistance and chromosomes was observed. The chromosomal character of a resistant subline was completely retained in the sensitive subline derived from it after 9 months of continuous cultivation in the absence of the drug. The folie acid reducÃ-asecontent increased with increasing resistance and was 300-400 times higher in the 3000-fold resistant cell line than in the sensitive parent cells. Extensive selection is shown to be involved in the development of the amethopterin-resistant cell lines. This selection is most probably based on the capacity of a cell to rapidly in crease the content of folie acid reductase.