Alveolar macrophages were lavaged from silica or saline instilled rats 0, 3, 7 and 14 days after exposure. Macrophages were cultured for twenty-four hours and the conditioned media assayed for the ability to stimulate rat lung fibroblast proliferation and collagen synthesis. Macrophages remained viable throughout the culture period. DNA synthesis was significantly elevated by macrophage conditioned media (MCM) from silica instilled rats (S-MCM) compared to untreated fibroblasts or fibroblasts exposed to MCM from saline instilled control animals (C-MCM). Stimulation of DNA synthesis was not observed when S-MCM was exposed to non-proliferating fibroblasts. Collagen synthesis quantitated as 3H-hydroxyproline accumulation or percent collagen synthesis was also increased by day 3 and day 7 S-MCM. Specific activity measurements of intracellular 3H-proline minimized the possibility that the increase in 3H-proline incorporation into collagen was a reflection of increased proline transport. Non-collagen protein synthesis was also increased in fibroblasts exposed to day 14 S-MCM. These results suggest that alveolar macrophages elaborate factors following silica exposure capable of altering the DNA and protein synthetic activity of lung fibroblasts. These changes in fibroblast DNA and protein metabolism are similar to those observed for lung tissue in vivo and lend further support to the hypothesis of macrophage mediation of the pulmonary response following silica exposure.