Summary. We demonstrate that osteoclast-like cells of GCT result from the spontaneous fusion and differentiation of CD14+ cells of the monoblastic lineage by an autocrine mechanism mediated by RANKL, rather than induced by stromal cells. This process is further enhanced by the simultaneous impairment of the negative feed-back regulation of osteoclastogenesis by interferon β. Introduction. Giant cell tumor of bone (GCT) is a benign osteolytic lesion with a complex histology, comprising prominent multinucleated osteoclast-like cells (OC), mononuclear stromal cells (SC), and monocyte-like elements. So far, most studies have focused on SC as the truly transformed elements that sustain osteoclast differentiation, while less attention has been paid on the monocyte-like cell fraction. On the contrary, we have previously shown that SC are non-transformed element that can induce osteoclastogenesis of monocytes at levels that do not exceed that of normal mesenchymal stromal cells. We therefore focused on CD14+ monocyte-like cells as an alternative key candidate for the pathogenesis of GCT. Methods. We isolated CD14+ enriched cell fraction from tumor samples by immunomagnetic separation. We analyzed CD14+ cells for ultrastructural morphology, mRNA levels of haematopoietic, monocytic, and dendritic markers, and for RANKL, and M-CSF. Due to the very high number of OC in GCT, we hypothesised that the IFN-b pathway might be impaired. In fact, IFN-b functions as a negative-feedback regulator that inhibits osteoclast differentiation. We assayed IFN-b mRNA and protein expression in both cultures and tumor samples. Finally, we verified the ability of CD14+ cells to spontaneously form osteoclasts. Results. In the CD14+ enriched fraction we identified two different cell populations, both positive for TRACP activity and negative for Ki-67 nuclear localization, one with an undefined histotype and the other showing characteristics of the monoblastic lineage, mainly monoblasts and promonocytes. Isolated cells were positive for CD45, MSE-1, RANK, CD14, and CD80, and negative for CD144, and were able to spontaneously form collagen-resorbing multinucleated cells, a process that was strongly impaired by the addition of osteoprotegerin. The expression of RANKL and M-CSF mRNA in cultured cells demonstrated the presence of an autocrine circuit inducing osteoclast formation. Finally, we found very low expression of IFN-b both in the in vitro formed OC and in tissue samples. Conclusions. These data show that CD14+ cells in GCT are monocyte-like cells that can spontaneously form bone-resorbing multinucleated cells through impaired IFN-b expression. Taken together, these data raise questions regarding the role of the CD14+ cell component and of their regulating mechanisms that may be relevant for the development of effective therapeutic strategies