A simple method is described which combines a sodium dodecyl sulfate polyacrylamide gel electrophoresis (SOS-PAGE) in the first demension with a second electrophoresis, at right angles to the first, into an agarose matrix. The proteins, separated by SDS-PAGE, are exposed to appropriate antisera after the second stage electrophoresis and immunoprecipitates form in the agarose corresponding to the relative electrophoretic mobilities of proteins in the first stage SDS-PAGE separation. The method thus provides a simple, reproducible means for correlating antigenicity with apparent molecular weight of proteins. The technique is qualtitative, but requires smaller quantities of antisera than more conventional immunoelectrophoretic methods such as rocket electrophoresis.