Three independent mouse monoclonal antibodies (mAbs) ID1 (IgG3), ID2 and ID3 (IgM) were raised against whole cells of a surgically resected human interdigitating cell sarcoma (ICS). In immunoperoxidase staining, these mAbs strongly stained the cytoplasm of ICS neoplastic cells as well as interdigitating cells in normal lymphoid tissues. These mAbs also detected monocyte/macrophages and dendritic cells, although their staining was highly variable depending on tissue distribution of the cells. Additional immuno-histological and enzyme histochemical study revealed that the neoplastic cells of ICS had cytoplasmic acid phosphatase and membranous alkaline phosphatase activity, and also possessed S100 beta protein, Ki-1 antigen. DAKO-macrophage antigen, and weak vimentin activity. Neither rearrangement of immunoglobulin heavy chain gene nor of T-cell receptor genes was detected in the DNA of ICS by Southern hybridization. These observations provide further confirmation of our previous finding (Nakamura et al. 1988, 1989) that the origin of ICS is interdigitating rather than lymphoid cell, and indicate that our mAbs could be useful as a cellular differentiation marker of interdigitating cells and for diagnosis of ICS.