SUMMARY The location of antigens on sections of bacteria using the soluble peroxidase- anti-peroxidase complex in conjunction with unlabelled antibody is described. Using this technique, spore antigens have been detected in the cytoplasm of vegetative cells during forespore septum formation and subsequent stages of sporulation. Antigenic sites were first associated with poly-beta-hydroxybutyric acid granules and subsequently were found in increasing quantities in the cyto- plasm of the sporangium. Vegetative cell antigens were located on the cell wall and in the cortical region during sporulation. During germination antigens were located in the cortical region, and during outgrowth on the cell wall. These findings are discussed in the light of existing biochemical data.