Parathyroid hormone-related peptide (PTHrP) induces pathological bone resorption in an endocrine manner, resulting in hypercalcemia of malignancy. However, the histopathological aspect of the action of PTHrP secreted by tumor cells on bone resorption has not well been documented. Therefore, we studied cell-cell interactions between bone cells, stromal cells, and PTHrP-secreting tumor cells (EC-GI-10) morphologically. Tumor cells injected subcutaneously into the parietal region formed a tumor mass, invading the bone marrow. The tumor mass was surrounded by a membrane structure consisting of stromal cells. These stromal cells were positive for alkaline phosphatase (ALPase). Tartrate-resistant acid phosphatase (TRAPase)-positive osteoclasts were localized close to the ALPase-positive cells, and numerous osteoclasts were observed on the neighboring bone surfaces. PTHrP, vascular endothelial growth factor (VEGF), and matrix metalloproteinase (MMP)-9 were detected in the tumor cells. Using RT-PCR, expression of interleukin (IL)-1Alpha, IL-1Beta, and PTHrP, which are strong bone resorption factors, was detected in the tumor cells. Some ALPase-positive cells localizing on the neighboring bone surfaces and endothelial cells revealed PTH/PTHrP receptor immunoreactivity. Ultrastructurally, numerous blood vessels were observed between the tumor nests and the stromal cells. The nests were surrounded by a basement membrane, but it was discontinuous, therefore permitting direct contact between the tumor cells and the stromal cells. These results indicate that PTHrP secreted by tumor cells appears to stimulate osteoclast differentiation and bone resorption in a paracrine manner through PTH/PTHrP receptor-immunopositive cells. IL-1Alpha, IL-1Beta, VEGF, and MMP-9 may also be involved in facilitating osteoclast formation and the subsequent bone resorption.