Adult STU mice with tumors induced by a progressor Moloney sarcoma virus transformant developed serum antibodies against juxtanuclearly arranged structures (JNS). 14 days after tumor cell transplantation the antibody activity against JNS could be observed. During repeated tumor transfers anti-JNS development always accompanied tumor proliferation. In indirect immunofluorescence microscopy JNS were recognized by these antibodies not only in cultured cells of murine origin (normal as well as transformed cells), but also in cells of other mammalian species (rat, bat, hamster, dog, goat, cattle, horse) and even in cells of members of two further vertebrate classes: avian embryo fibroblasts and a fish-derived cell line. In the various cell lines tested, the morphology of the JNS differed, but they were always arranged close to the nucleus. The distinct perinuclear and juxtanuclear position is indicative for the Golgi region; the staining pattern after osmium impregnation and immunoelectron microscopy supported the localization of the recognized structures in this cellular compartment. Drugs like monensin or colcemid, which are known to influence Golgi morphology and function, altered the staining pattern of the antibodies drastically. The widespread occurrence of this antigenic determinant(s) in cultured cells from different species suggests that this Golgi component is highly conserved in evolution. According to immunoprecipitation studies four proteins with apparent molecular weights of 250,000, 96,000, 53,000, and 38,000 daltons were recognized by these antisera. It remains to be determined, if these molecules are involved in the described serological reaction.