Ten cases of "undifferentiated" large-cell tumors were ultrastructurally characterized by cells with abundant filiform cytoplasmic projections without intercellular junctions. These cases were studied by means of the avidin-biotin-peroxidase complex (ABC) technique applied to formalin-fixed, paraffin-embedded sections using antibodies against high- and low-molecular weight keratins (Ker), vimentin (Vi), epithelial membrane antigen (EMA), S-100 protein, leucocyte common antigen (LCA), kappa (K) and lambda (L) light chains, Leu-M1, lysozyme (Ly), alpha-1 antitrypsin (A1AT) and alpha-1 antichymotrypsin (A1ACT). All 10 cases were negative for Ker and EMA but positive for Vi. S-100 was present only in scattered dendritic cells. LCA was identified in seven cases. In the three LCA-negative cases, two stained for Leu-M1, and one of these also showed intracytoplasmic L; one was negative for all markers but Vi. None of the tumors showed any significant staining for Ly, A1AT, or A1ACT. Our findings indicate that these tumors are nonepithelial and nonneuroectodermal, and that they are best classified as non-Hodgkin's lymphomas. The possibility that some of the filiform large-cell lymphomas may be derived from dendritic reticular cells cannot be excluded.