The preparation of glutaraldehyde fixed lymphocytes as a reference standard in flow microfluorometry is described. The ease of preparation, the stability of the cells over time, their ability to stain reproducibly in the range expected of fresh human peripheral blood and bone marrow cells and, most importantly, the fact that they may be used to resolve optimally and situate correctly the G1 peak of normal mammalian cells, make them a reliable tool for the calibration of flow microfluorometers and for detection of aneuploid populations.