Analysis, and evolution with time in culture, of metaphases of MDCK (A,B), MDCK I (C-E) and MDCK II (F-J) cells. Cells obtained from reliable sources (Table 1) were maintained continuously in culture for 2-3 months. The number of chromosomes of 50 metaphases was determined upon their receipt and after various periods of time in culture. Insert in J: phase contrast image of cells maintained for three months in culture. Note the heterogeneity of the cell population, composed of two types of cells: large and flat cells on the right and small and tight cells on the left. This latter type was observed only after two months in culture. Bar, 10 μm.