Twenty-one laboratories participated in a baseline study of their ability to agree on the measurement of fluorescence intensities of a stable multi-peak reference material (FlowCal 575) and of stained and fixed CD4 lymphocytes. Relative fluorescence intensities were calculated as ratios to the most fluorescent bead. The good correlation between laboratories suggests that this simple approach may be useful in multi-laboratory studies. The data also provide a baseline for the evaluation of any improvement of inter-laboratory agreement gained by more rigorous and demanding approaches.