We investigated the expression of tumor necrosis factor-alpha (TNF alpha) gene in the area of implanted calf cancellous bone with different calcium content and the influence of calcium salts on local cellular immunity. The particles of mineralized and partially demineralized bone were implanted in the mouse's muscle pouch, and removed 3, 5, 10 and 20 days after implantation of the bone particles. The specimens were processed for determining the expression of mRNA encoding TNF alpha, which was performed by a nonradioactive in situ hybridization technique. The expression of TNF alpha mRNA was markedly higher in the mineralized bone group than in the partially demineralized bone group (P < 0.01). The positive rate of TNF alpha gene expression was highest by 10 days after implantation. There was a strong hybridization signal localization to the cytoplasm of morphologically identifiable monocytes and multinucleated giant cells. Similar activity was detected in the cytoplasm or nuclei of mesenchymal cells, fibroblasts as well as striated muscle fibers. This finding suggests that the calcium content in calf cancellous bone possesses a significantly stimulative effect on TNF alpha mRNA expression, and calcium salts may be of importance for the modulation of local cellular immunity.