Differential is a registered trademark for a series of photometers, measuring differential light scattering which are produced by Science Spectrum, Inc., Santa Barbara, California. Diff III, a shortened form of Differential III, is also a trademark of the firm. The new Differential III employs a 15-element detector array to measure monochromatic coherent vertically polarized light scattered as a function of angle. Differential light-scattering techniques offer the unique capability to measure certain characteristics and changes in single bacterial cells which appear to be sufficiently distinctive to identify certain bacteria. The newest form of Differential III is intended primarily for clinical laboratory use for antibiotic susceptibility testing and will be described in the most detail. The Differential III is equipped for data input and program storage on a small computer disc and is programmable in BASIC for the clinical user. The instrument requires the specially configured cuvettes designed for Differential III.