Nineteen giant cell tumors of tendon sheath (GCTTS) were studied to elucidate the origin of the proliferating cells of these tumors, using single and multiple immunostaining techniques with a labeled avidin-biotin [LAB] method in paraffin-embedded tissues. Proliferating cell nuclear antigen (PCNA) and Ki-67 (MIB-1) antigen were present in mononuclear cells (PCNA 26%; MIB-14%) but were absent in giant cells. These findings indicate that mononuclear cells, but not giant cells, participate in the proliferative compartment of GCTTS. A histiocytic marker, HAM56, was positive in many mononuclear cells (mean 81%), but was totally negative in osteoclastic giant cells. Another histiocytic antigen, CD68, was expressed in both mononuclear cells (mean 28%) and most of the giant cells (mean 89%). By triple immunostaining for PCNA, HAM56 and vimentin, 83% of PCNA-positive mononuclear cells co-expressed HAM56. Because of the frequent co-expression of PCNA and HAM56, the main portion of proliferating cells in GCTTS may represent a monocyte/macrophage lineage. However, there is a small but definite mesenchymal/fibroblastic component, characterized by PCNA+vimentin+HAM56-, relating to the proliferative compartment of GCTTS. Multiple immunostainings with MIB-1 showed similar patterns to those with PCNA. These observations indicate that the GCTTS represent bimodal proliferative lesions consisting of histiocytic and mesenchymal/fibroblastic elements.