A rapid testing is required in clinical laboratory. A new method for rapid bacterial testing is proposed by size distribution measurement with laser light scattering. Bacterial sample is suspended in a saline solution and the scattering pattern is measured from 5 to 90 degrees with 5-degree intervals. The size distribution function is calculated from the light scattering pattern by the inversion technique based on the modified Rayleigh-Debye approximation. The difference in bacterial species and the influences of drug are investigated. The following results are obtained; (1) size distribution curve reflects the morphological characteristics of bacteria, and bacterial species are discriminated roughly by two characteristic parameters (mean size and dispersion width) obtained from the curve; (2) size distribution pattern changes in a short time (within one hour) indicating the effects of drug. It is useful for antibiotics susceptibility testing.