Appropriate concentrations of carrageenan and antibody to carrageenan were mixed in a medium consisting of 0.1% agarose in 0.01 M phosphate-buffered saline pH 7.4 at 37 degrees C. The kinetics of precipitation were followed by turbidimetric analysis with a double beam Unicam spectrophotometer set at 420 nm. Addition of agarose enhanced the antigen--antibody reaction as judged by turbidity and served as a stabilizing medium permitting extended periods of continuous monitoring. The initial rate of precipitation was found to vary linearly with the concentration of antigen in the antibody excess zone. Both the critical rate of precipitation and final optical density were characteristic of the specific antigen-antibody reaction. These two parameters are used to quantitate the extent of a cross-reaction and to calculate an index of homology for the cross-reacting antigen to the homologous antigen.