Although cell polyploidization is not an infrequent event in mammalian cells and is common in tumours, the mechanisms involved are not well understood. Using the murine B16 cell line as a model, we evaluated the role of some key proteins involved in cell cycle progression: p34(cdc2), cyclin B1 and PCNA. By means of flow cytometry, we showed that both in modal- and in high-ploidy subpopulations, almost all cells were p34(cdc2)-positive. In the modal-ploidy subpopulation only 17.1% cells were cyclin B1-positive and 85.6% PCNA-positive; in contrast, in the high-ploidy subpopulation up to 91.8% cells were cyclin B1-positive and 97.3% cells were PCNA-positive (P < 0.001). Immunofluorescence microscopy showed that PCNA was located in the nucleus; p34(cdc2), both in the nucleus and cytoplasm; and cyclin B1 yielded a cytoplasmic spotted pattern with a perinuclear reinforcement. After a 24-h incubation with 3[H]-thymidine followed by withdrawal of the isotope, high-ploidy cells remained labelled 8 days after thymidine withdrawal, in contrast to modal-ploidy cells. Taken together, our results suggest that polyploid cells are not quiescent, their cell cycle is longer than that of the modal-ploidy population, and they maintain cyclin B1 throughout the cycle, which may contribute to their genesis by impeding the exit from mitosis.