The main components of an unusual form of lung tumor were osteoclast-like multinucleated giant cells and mononuclear stromal cells. Besides, scattered islands of moderately differentiated squamous cells also appeared. Both the mononuclear and the osteoclast-like giant cells reacted with antibodies against CD68 and vimentin, but did not react with antibodies against cytokeratin, EMA and CEA, or lysozyme and α-1-antitrypsin. The p53 and PCNA antigens were positive only in mononuclear cells and not the osteoclast-like giant cells, suggesting that mononuclear cells represent proliferating elements with histiocytic differentiation while osteoclast-like giant cells are stromal, presumably reactive components of the tumor.