A small proportion of mouse ascites fluid induced by hybridomas producing monoclonal antibodies or myelomas secreting immunoglobulin yielded staining that was confined to the Golgi zone of certain epithelial cell types in rats and gerbils but not in mice. In addition, a commercial IgG fraction from mouse plasma similarly labeled the Golgi area, unlike IgG from mouse serum from another source. Culture supernatant from one hybridoma line contrasted with ascites fluid produced by the same hybridoma in failing to stain the Golgi region. The capacity of a fluid to react with the Golgi cisternae bore no relationship to the class of immunoglobulin secreted by the hybridoma or myeloma. Absorption of an ascites fluid with blood group A1 human erythrocytes eliminated its affinity for Golgi cisternae. Adsorption with blood group A2 or B or two type O cells used for screening for blood group antibodies had no effect on Golgi zone labeling by this ascites fluid. The positive cells included most serous secretory cells in rats, serous cells of sublingual and tracheal glands, and some endometrial and oviduct-lining cells in gerbils, and columnar lining cells of small intestine and cecum and all or part of the lining cells in some prostate lobes in both genera. Some of the tested ascites fluids stained mast cells. The agent accounting for mast cell labeling differed, however, from that reacting with Golgi cisternae in its distribution among the mouse ascites fluids examined, lack of relationship to the ABO blood group system, occurrence additionally in normal rat serum, and capacity to stain cells in mice as well as rats and gerbils.