We have characterized a population of large, brain-theta antigen-positive, secretory cells that occur in high frequency among murine bone marrow (approximately 6%), spleen (approximately 0.5%), and fetal liver (approximately 5%) cell populations. They do not occur in significant numbers among lymph node, thymus, and peritoneal exudate cells. These cells are Ly1, Ly2, and Thy1.2 negative, adherent to nylon wool, Sephadex G-10, and plastic, but peroxidase and Sudan black negative. They are radiation, cyclophosphamide, and cortisone resistant. These cells secrete a product of 30,000 to 40,000 m.w. of unknown function. Production of antisera specific for this molecule has facilitated identification of the cell by using a reverse hemolytic plaque-forming cell assay. Possible functions of this cell and its product are discussed.