Flow-cytophotometers are optical electronic devices which measure a variety of physical characteristics of individual cells in suspension as they flow, one at a time, past a laser beam of a particular wavelength. Some instruments measure fluorescent emission at different wavelengths and light scattered from each cell detected at a narrow angle. Other instruments measure both narrow angle light scatter and axial light-loss (extinction) due to impingement of the cell on the light path as it traverses the laser beam. The studies to be described will refer only to instruments of the latter type.