Summary A technique called direct immunoelectrophoresis is described, by which it is possible to visualize the size, shape, position and homogeneity of an entire antigen spot after electrophoresis in agar, by flooding the surface of the agar with antibody. The appearance of the visualized spot is similar to that seen when a protein substance is stained directly with dyes in simple agar electrophoresis. Protein antigens in too low concentration to be demonstrated by staining may be seen by direct immunoelectrophoresis, as may unstainable nonprotein antigens. Physicochemical heterogeneity of immunologically related antigenic substances can, in some cases at least, be demonstrated better by direct than by standard immunoelectrophoresis.