Immunologic responses were determined in Balb/c mice following intermittent silica or olivine inhalations for 150, 300, or 570 days. Animals dust-exposed for 570 days were tested immediately postexposure, while those exposed for 150 or 300 days were tested immediately or were rested for 30 or 150 days as a measure of possible recovery from effects of the dust inhalations. Silica inhalation suppressed the number of specific plaque-forming cells (PFC) in the spleen produced in response to aerosolized Escherichia coli bacteria. When tested after 570 days, silica inhalation also reduced the ability of alveolar macrophages to phagocytize Staphylococcus aureus in vitro. Olivine inhalation also suppressed splenic PFCs and alveolar macrophage phagocytosis, but to a lesser degree than silica. In animals tested after 570 days of dust exposure, it was determined that the ability to lyse allogeneic tumor cells in vitro was impaired by olivine slightly more than by silica, while antibody-dependent cell-mediated cytotoxic and mitogenic responses by splenic lymphocytes were unchanged by inhalation of either dust. The effects of increased exposure periods, and of recovery periods after exposure, were confounded by age-related immunologic changes which were present after the longer exposures.