Summary— A new method of measuring urinary particulate activity is described. The analytical process, using a Coulter Counter, is designed to have minimal effect on the dynamic nature of the specimen. Emphasis is placed on conducting the analysis during the first 5 min following voiding, preventing changes in temperature and pH and avoiding any form of dilution of the specimen. Urinary particulate activity is measured by quantitatively evaluating the spontaneous changes which occur in particulate number and size. Resulting values demonstrate clearly defined differences between urine specimens which appear biochemically similar. The results are reproducible under stable conditions but are significantly altered by temperature and pH change, or dilution of the specimen. This method measures the urinary potential to form precipitates and ultimately stones, and should be included in any investigation of urinary calculus disease.