Near DNA diploid human adult solid tumors are often associated with certain near-tetraploid cells. In an established human breast cancer cell line, Hs578T, with a DNA index in the hyperdiploid region, polyploid cells appeared during exponential growth. Among clones generated from single cells and analyzed by the video time lapse technique, an intraclonal interdivision time (IDT) heterogeneity is presented that renders endoduplication a plausible explanation for the generation of the polyploid cells observed. This conclusion, drawn from our IDT analysis, is supported by curves drawn from counting grain-positive cells during continuous labeling with [3H]-thymidine. Our results are compared with a parallel analysis of the aneuploid human breast cancer cell line MDA-231, generating intraclonal IDT heterogeneity, due mainly to the mitotic instability of that line, as we reported previously.