Two distinct subpopulations of cells, light (L-cells) and heavy (H-cells), have been isolated and characterized from a rat ascitic tumor, the Zajdela ascitic hepatoma. These two populations have been separated by Percoll density gradient centrifugation and studied by flow cytofluorimetry. The two populations, in addition to their difference in buoyant densities, show characteristically different profiles for DNA and RNA contents, nonspecific esterase activity, and surface amino group distribution. The DNA distribution in the two types of cells clearly shows that the H-cells are rapidly proliferating while the L-cells are quiescent. Studies on the two groups of cells after colchicine treatment also confirm this conclusion. The H-cell induced tumors kill the host animals rapidly while the L-cell induced tumors regress in about 3 months. The H- and L-cells from the Zajdela tumor form a convenient experimental system to study the marked difference in the progression of tumors induced by these cells, possible differences in gene expression in regressing and nonregressing tumors and the interactions between the subpopulations with a view to delineate molecular events governing tumor progression and tumor heterogeneity.