Serum applied as a streak to a strip of filter-paper can be separated into six main bands, depending upon respective molecular weights (electrophoresis). When such bands of protein are exposed to a polyvalent anti-human serum diffusing in at right angles, at least 18 protein components can be identified as precipitation arcs (immunoelectrophoresis).Using immunoelectrophoresis, 202 sera taken at intervals from 86 patients with tuberculosis were compared with 160 sera from healthy persons and patients with non-tuberculous disease. With clinical recovery from tuberculosis, shrinkage of the beta-2 macroglobulin band was observed, and a gamma-1 globulin arc appeared or was accentuated. This finding suggests an early antibody response of large molecules, and a later defence by slowly developing small molecules, as seen in many common infections. The macroglobulins involved were found to be selectively adsorbed on attapulgite clay particles. This technique should be useful in developing serological tests for active tuberculosis.The apparatus used was designed and built by the authors at trivial cost. Its construction and operation are described.