Methods for combined cytofluorometric determinations of nuclear DNA plus other parameters (proteins, enzyme activity, or immunoreactive substances) are reported. DNA measurement was performed on an autoradiograph of Feulgen-stained fetal mouse hepatocytes. In order to facilitate the multiparametric measurements and data processing, a fluorescence cytophotometer (BH2-QRFL, Olympus, Tokyo) equipped with four interchangeable filter-sets and controlled by a desktop computer (HP-85F), was used. All data stored in the same computer were displayed in a two-dimensional scatter plot or two-dimensional frequency distribution histogram. Analysis of the DNA histogram of fetal mouse hepatocytes was accomplished by the same computer. Calculated fraction sizes for G1, S, and G2 plus M phases were found to be in good correlation with those determined directly on the simultaneously combined tritiated thymidine (3H-TdR) autoradiography.