Three cases of infantile digital fibromatosis were studied by electron microscopy, enzyme histochemistry, and tissue culture. The tumors were made up equally of myofibroblasts containing electron-dense inclusions which were composed chiefly of microfilaments measuring about 5 to 7 nm. Dense bodies usually observable in the smooth muscle cells were found in the bundles of these microfilaments and in the process of the inclusions, suggesting that these inclusions may represent an abnormal accumulation of contractile protein in the cytoplasm of tumor cells. Two cell lines were established from culture of the tumor cells, and the cultured cells also contained inclusion bodies showing the same morphologic characteristics as those of the original tumor cells. Lysosomal enzymes were abundant in the cultured cells, but they were scant in the cells of the fresh tissue specimens. Cocultivation of the cultured cells with human embryonic lung cells yielded no cytopathic effect.