The chromosome number of the Krebs-2 ascites carcinoma maintained in Swiss mice was found to be grouped about a modal number of 78–79. After in vitro culture in medium No. 858 (modified) containing 10 per cent calf serum, primary cultures showed a shift to a smaller number of chromosomes. This medium when used for originating single-cell cultures favored the selection of cells with a modal chromosome number of 64–65. The cell lines selected by the medium were, for the most part, stable with respect to maintaining the same modal number both in vitro and in vivo.