Fig. 1. Supernatants from silica-treated macrophages induced the proliferation of NIH-3T3 and MRC-5 cells. Raw264.7 cells were treated with different concentrations of silica for 24 h and the supernatants were used to culture fibroblasts. NIH-3T3 cells were cultured in DMEM (NC) or the supernatants collected from Raw264.7 cells exposed to 0, 25, 50, 100 or 200 μg mL^–1 silica solution for 24 h. MRC-5 cells were cultured in DMEM (NC) or the supernatants collected from Raw264.7 cells exposed to 0, 12.5, 25, 50 or 100 μg mL^–1 silica solution for 24 h. The MTT assay was performed to measure cell proliferation. In the case of NIH-3T3 (a), the peak value and significant proliferation compared with the NC group were shown in the silica 100 μg mL^–1 group, while for MRC-5 (b) these appeared in the silica 50 μg mL^–1 group. The results show data from five repeated tests. The data are presented as means ± SD. The significance of differences was determined by ANOVA followed by the Dunnett's test; *p < 0.05 versus the NC group.