The Serum Agar Measuring Aid (SAMA) is an accessory device for the Photronreflectometer. The combination permits photometric quantitation of serum agar column reactions. Results and evaluation indicated that accurate measurement of the density and diffusion of zones of precipitation can be made by beginning laboratory technicians. SAMA can also be used to quantitatively determine antibody response, follow protein fractionation, and identify unknown participants in precipitin reactions.