The biological effects of ore dusts were examined on peritoneal macrophages in vitro and on rat lung in vivo. The methylene blue adsorption of the dust samples, cytotoxicity, and phospholipid and hydroxyproline content of the lung were determined. It was realized that the examined ore samples had cytotoxic effects and from the in vitro results one can get information about the type of damage to be expected in the lung tissue. It is supposed that the examined ore dusts are less dangerous and slower acting than quartz, but they cause progressive fibrosis. The in vivo biochemical experiments do not give information about the type of damage to be expected in lung tissue; and the data on ore, bentonite, and coal do not differ from each other. To determine the typical biochemical changes caused by quartz, we obtain the most valuable information by determination of lung weight and phospholipid, phosphatidylcholine, and hydroxyproline content.