A histiocytic sarcoma was present at birth in a pig. On the basis of ultrastructure and structural-protein composition (presence of a-smooth-muscle actin but not keratin), the sarcoma component was identified as a leiomyofibrosarcoma. Lipid-laden macrophages (histiocytes), which permeated the tumour in an apparently random fashion, were somewhat atypical in that they were negative for some macrophage markers; they gave a reaction, however, for CDw14. Despite its aggressive metastatic capacity, this tumour occurred almost exclusively in the subcutis, dermis and skeletal muscle. The tumour was extensively vascularized with many small capillaries which did not express E-selectin (CD69E), MHC class II or the L-selectin (CD69L) ligand, markers characteristic of inflamed (activated) endothelial cells in pig skin. Significant numbers of the histiocytes were positive for the integrins CD18 and VLA-4 (CD49d), indicating involvement of integrin pathways in the spread or growth, or both, of the leiomyofibrosarcoma. Most of the fibrous sarcoma cells also had extensive reactivity with an antibody to the standard variant form of CD44 (CD44s). (C) 1996 W.B. Saunders Company Limited