Some authors have claimed that actin is not immunostained in characteristic intracytoplasmic inclusions of infantile digital fibromatosis, whereas others have claimed that it is. Formalin-fixed specimens were used in the former studies; specimens fixed in alcohol used in the latter studies. Actin at other sites, such as the rim of the inclusions, was distinctly immunostained even in the formalin-fixed specimens. Such phenomena make it difficult to accept the loss of antigenicity of actin as a result of formalin fixation. The use of usual pretreatment with trypsin provided the same results. We were able to immunostain actin distinctly and strongly in the inclusions for the first time in formalin-fixed specimens by combining KOH in 70% ethanol and trypsin. This successful staining results from the adequate etching effect of trypsin, which occurs because of a loosening of proteins in the inclusions due to KOH. These phenomena suggest that steric hindrance of antigen determinant has occurred only in the inclusions as a result of intramolecular cross-linkage, because of extremely dense accumulation of actin filaments in the inclusions.