Dunning R3327-H rat prostate adenocarcinoma cells, when grown in syngeneic (Copenhagen) rats or nude mice, produce tumors with prominent hypercellular stroma. The authors have previously demonstrated the presence of anomalous steroid-sensitive cells in both the epithelium and stromal compartments of this model system. In order to better understand the histogenesis of these cells, the authors studied samples of the tumor which were radiolabeled overnight with tritiated dihydrotestosterone (3H-DHT). Frozen sections of the tissues were thaw-mounted onto autoradiographic emulsion-coated slides to permit silver grain identification in association with nuclei of androgen-sensitive cells. Surprisingly, numerous silver grains were found to be associated with nuclei of large cells within the stroma. Therefore, these cells were termed 'epithelioid' pending confirmation of their origin. To further define these cells and their relationship to the surrounding matrix, autoradiograms have now been examined immunohistochemically with antibodies directed against the basement membrane glycoprotein, laminin, as well as antibodies specific for intermediate cytoskeletal filaments. Following identification of acinar basement membranes, epithelioid cells were identifiable both in the stroma and in the acinar epithelial cell layer. Histochemical staining with acid phosphatase, a marker for prostatic epithelium, was performed and shown to be present in acinar epithelial cells as well as in epithelioid cells. Additionally, fluorescence-activated cell sorting was employed to characterize the DNA content of cell types within the H tumor. Epithelioid cells were found to be in highest concentration in an aneuploid peak with a ploidy of approximately 6N. The autoradiographic, immunohistochemical, cytometric, and ultramicroscopic studies suggest that 1) epithelioid cells are epithelial derived stromal cells; 2) these epithelioid cells arise by pathologic division of aneuploid neoplastic precursor cells of approximately 3N ploidy, which are found within the prostatic epithelium; and 3) the resulting 6N cells degrade the basement membrane locally, invade the stroma, and populate it. Here, they can be distinguished from fibroblasts by their size, acid phosphatase activity, and hormone receptor content. Thus, the term 'epithelioid' is inappropriate; and these cells should be regarded simply as large neoplastic epithelial (LNE) cells. The presence of this cell type suggests that this tumor subline represents a useful naturally occurring model for the study of the initial stages of neoplastic transformation.