Multinucleate cells were induced by colcemid treatment in PtK1 cells in culture. DNA synthesis and mitotic behavior of those cells in which each nucleus contained a single chromosome were studied. More than 80% of such cells showed synchronous DNA synthesis and mitosis in all nuclei. As these genetically different nuclei respond identically to the molecules that initiate DNA synthesis and mitosis, intranuclear control of initiation of DNA synthesis and induction of mitosis by genes on individual chromosomes can be excluded. The occasional occurrence of asynchronous division in multinucleate cells is assumed to result from unequal availability of the inducer molecules to the individual nuclei.