A cytochemical method was developed to differentially stain cellular DNA, RNA, and proteins with fluorochromes Hoechst 33342, pyronin Y, and fluorescein isothiocyanate, respectively. The fluorescence intensities, reflecting the DNA, RNA, and protein content of individual cells, were measured in a flow cytometer after sequential excitation by three lasers tuned to different excitation wavelengths. The method offers rapid analysis of changes in the cellular content of RNA and protein as well as in the RNA-protein, RNA-DNA, and protein-DNA ratios in relation to cell cycle position for large cell populations. An analysis of cycling cell populations (exponentially growing CHO cultures) and noncycling CHO cells arrested in the G1 phase by growth in isoleucine-free medium demonstrated the potential of the technique.