The measurement of anti-D concentration has been carried out by two immunoprecipitation techniques utilizing sonicated D-antigen-containing stroma suspended in agarose. One method utilizes single radial immunodiffusion; in the other method, anti-D is moved into the agarose by electrophoresis. The results obtained form both immunoprécipitation techniques were compared to those obtained by a standard 125Iantiglobulin method. The coefficient of variation of the difference between the results obtained by immunoprécipitation and those obtained from the 125I-antiglobulin method were ± 14% for the electrophoretic method and ± 16% for the radial diffusion method.