Cells of leaf explants of a monohaploid potato (Solanum tuberosum) were stimulated to mitosis on a medium with 5-bromodeoxycytidine during a period of 7 days. The cells cycled with mono- or diplochromosomes which showed differential staining of the sister chromatids and sister chromatid exchanges by the fluorescent plus Giemsa technique after two rounds of BrdC incorporation. Through the staining pattern the course of the first three cell cycles could be traced and the duration of the cycles estimated. Polyploidisation was enhanced by selective stimulation of polyploid cells and by endoreduplication of G2-phase cells. The percentage of polyploid mitoses increased from 10 to 70.