The immunoelectrophoretic patterns of plasma proteins from mice are altered significantly by acute infections. Some proteins are dissociated into two or more components, some showed striking increase in plasma concentration, others are depleted, and certain ones appear which are undetectable in normal samples. beta(1)-C dissociated into two electrophoretic components under a variety of conditions in addition to infections. Endotoxins and killed organisms in vivo, and specific precipitate absorption, heat and aging in vitro produced this change. Endotoxins injected into mice also induced a rise in haptoglobin though not as sharply or predictably as acute infection. Preliminary results with samples from hospital patients with acute diseases are discussed. It was concluded that study of experimental diseases in laboratory animals by these techniques could provide a fruitful basis for the investigation of the plasma protein changes in similar human diseases.