Sirishinha, Stitaya (University of Rochester School of Medicine and Dentistry, Rochester, N.Y.), and Peter Z. Allen. Immunochemical studies on alpha-amylase. III. Immunochemical relationships among amylases from various microorganisms. J. Bacteriol. 90:1120-1128. 1965.-Immunochemical relationships among amylases obtained from a selected group of microorganisms were examined, and a cross-reaction was detected between the alpha-amylases of Bacillus stearothermophilus and B. subtilis. Immunodiffusion and quantitative precipitin studies, as well as cross-neutralization tests, indicate that B. stearothermophilus alpha-amylase reacts with a portion of antibody present in antisera to crystalline B. subtilis alpha-amylase. Amylases from these two species thus have some aspects of structure in common. Limited data obtained by immunodiffusion suggest that groupings which confer cross-reactivity to the B. stearothermophilus enzyme are lost after exposure to mercaptoethanol in the presence of ethylenediamine-tetraacetate, followed by treatment with iodoacetamide. With the antisera employed and within the concentration range examined, no immunochemical cross-reaction was observed among amylases from Aspergillus oryzae, B. subtilis, B. polymyxa, B. macerans, Pseudomonas saccharophila, and Euglena sanguinis. Immunoelectrophoresis of partially purified B. stearothermophilus alpha-amylase by use of antiserum to the crude enzyme, together with localization of amylase activity in immunoelectrophoretic plates, suggests that B. stearothermophilus alpha-amylase is antigenic in the rabbit.