A flow cytometer capable of measuring fluorescence lifetimes by the phase shift method has been built and evaluated. Under optimal conditions, the resolution of the fluorescence lifetime measurement is shown to be under 200 picoseconds. Pulse intensity variations are normalized using limiting amplifiers and electronic filtering. Normalization of signal intensities provides a lifetime measurement that is independent of fluorescence intensity over at least a 50-fold (17 dB) range in fluorescence intensity. The fluorescence lifetimes of unbound dye, fluorescent beads, cells stained with ethidium bromide, propidium iodide, and phycoerythrin-conjugated monoclonal antibodies have been measured. The fluorescence lifetimes measured for these particles are well correlated with lifetime measurements made using a standard fluorimeter. Cells stained with ethidium bromide and propidium iodide at various nucleotide-to-dye ratios are shown to exhibit similar behavior to static cuvette measurements. The fluorescence lifetime parameter is also shown to resolve phycoerthyrin fluorescence from propidium iodide fluorescence.