In vitro cell culture techniques were used to identify and characterize the individual cell types present in human giant cell tumors of bone. Three major cell types were identified based on morphologic characteristics, patterns of specific cell surface antigens, presence of receptors for hormones, and cell products. One population consisted of mononuclear cells that expressed monocyte-macrophage markers. These cells lacked receptors for skeletal hormones and did not persist in culture. Distinct from these cells was a population of mononuclear cells that proliferated in culture and most likely represented the neoplastic element of the tumor. These cells phenotypically resembled connective tissue stromal cells, i. e., they did not express macrophage surface antigens and they produced collagens (Types I and III). They also possessed receptors for parathyroid hormone. The final population of tumor cells consisted of multinucleated giant cells. These cells lacked monocyte-macrophage surface antigens and possessed receptors for calcitonin, a phenotypic marker for osteoclasts. These studies illustrate that in vitro cell culture techniques can be employed to identify and characterize the cell populations of tumors of skeletal tissues, and demonstrate the usefulness of these cell culture models for investigating the biology of bone tumors.