Cell cultures were carried out from four patients with infantile digital fibromatosis. The cultured cells, which contained cytoplasmic inclusions identical to those of the original tumor cells, possessed cortical bundles of microfilaments, rich network of granular endoplasmic reticulum, and well-developed Golgi complex. To demonstrate the distribution of actin filaments in the cultured cells, the heavy meromyosin-binding method was applied to saponin-treated cells from one of the four patients. The microfilaments constituting the inclusions as well as the cortical bundles were decorated with heavy meromyosin, presenting the "arrowhead complexes" specific for actin filaments. The inclusion may represent abnormal contraction of actin filaments in the cytoplasm of myofibroblasts.