Radial Immunodiffusion (RID) is a quantitative immunodiffusion technique used to determine the concentration of an antigen in a biological fluid. This method is based on the diffusion of an antigen in a radial pattern from a cylindrical well through an agarose gel containing an appropriate mono-specific antibody. As the antigen diffuses radially in all directions into the agarose, its concentration continuously decreases until the equivalence point is reached. At this point an immune precipitation occurs as the antigen and antibody reach equilibrium, the result is the formation of a precipitin ring. Since the diameter of the ring is directly proportional to the antigen concentration, a calibration curve of diameter versus antigen concentration can be constructed using a series of reference standards of known antigen concentration. The concentration of the antigen in an unknown sample is then determined by the measurement of the ring diameter produced by that unknown sample and reading the corresponding concentration values from the calibration curve.