In order to clarify the relation between histological classification and biological behavior of individual tumor cells, we quantitatively analyzed the cell kinetics of 56 advanced gastric cancers using DNA-RNA cytofluorometry after acridine orange fluorescence staining and compared the results with histological findings. The gastric cancers examined could be classified into two main groups based on the ploidy patterns constituting the cell population kinetics. In group I, tumor cells were found to be chiefly composed of diploid cells with many S-phase cells even at their advanced growth stages, and their morphological characteristics were shown to correspond to undifferentiated carcinoma (according to Nakamura & Sugano). In group II, tumor cells were composed of various classes of polyploid cells as well as of diploid cells with an increased number of S-phase cells, and the extent of polyploidization appeared to progress with the growth of the tumor. The morphological characteristics of this group tended to correspond to differentiated adenocarcinoma. It is, therefore, concluded that both the histological structures and cell morphology have close relationships with cell kinetics in the human gastric cancers.