The in vitro interaction of macrophages with ingested quartz led to the formation of a factor which when applied to fibroblasts stimulated their production of hydroxyproline without evidence of proliferation, an effect which titanium dioxide failed to induce. Inhaled quartz, however, also led to a pronounced degree of alveolar lipo-proteinosis, the lipid component of which, when introduced parenterally into other animals, stimulated proliferation of the monocytic line of marrow cells, from which macrophages are derived and populate the lung. The interrelation and implications of these two aspects of the macrophage response to silica are outlined.