Abstract: A differential light scattering photometer is disclosed for testing samples of bacteria in suspension. A plurality of cuvettes containing the samples are placed in a cuvette holder or carousel which rotatably places each cuvette in position to interrupt a laser beam. A detector rotates in an arc about each sample to produce output signals representing the scattered light intensity as a function of angular location, relative to the incident laser beam, about the sample. A control differential scattering pattern is produced and compared with test differential scattering patterns derived from test samples having added antibiotics. The patterns are compared by data processing to produce an output punched card indicative of bacterial sensitivity to each antibiotic. The data processor is responsive, through pattern comparison, to various responses of the bacteria, i.e., whether the antibiotic is bactericidal, bacteriostatic or the bacteria actually metabolize the antibiotic.