Several subcompartments of the cell cycle in addition to the G1-, S-, and G2-phases usually observed were identified by simultaneous flow cytometric measurements of ethidium bromide fluorescence and side scatter intensity of cell nuclei. Metaphase cells and very early G1-phase cells (G1A) with low side scatter intensities were discriminated from interphase cells with high side scatter intensities. The reason for the various side scatter intensities was found to be the different structure of metaphase cells and early G1-phase cells due to chromatin condensation as shown by sorting of the respective cell nuclei. The G1A-phase could further be subdivided into two compartments with very low side scatter (G1A1) and intermediate side scatter (G1A2) intensities. Using partially synchronized cells the duration of these subcompartments of the G1-phase could be estimated. The durations of G1A1- and G1A2-phases were found to be about 10 min and 20 min, respectively, compared to the total duration of the G1-phase of about 3 h. Additional flow cytometric measurements of side scatter intensities of cell nuclei provide therefore further information on subcompartments of the G1- and G2/M-phases.