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Asbestos is a group of naturally occurring mineral fibers which are associated in occupational settings with increased risks of malignant mesothelioma (MM), lung cancers, and pulmonary fibrosis (asbestosis). The six recognized types of asbestos fibers (chrysotile, crocidolite, amosite, tremolite, anthophyllite, and actinolite) are different chemically and physically and may have different dose-response relationships in the development of various asbestos-associated diseases. For example, epidemiologic and lung fiber content studies suggest that the pathogenic potential and durability of crocidolite is much greater than chrysotile asbestos in the causation of human MM. We have used isolated mesothelial cells, the target cells of MM, as well as epithelial cells of the lung, the target cells of lung cancers, in vitro to elucidate the dose-response relationships in expression of early response protooncogenes and other genes critical to cell proliferation and malignant transformation in cells exposed to crocidolite and chrysotile asbestos, as well as a number of nonpathogenic fibers and particles. These studies reveal distinct dose-response patterns with different types of asbestos, suggesting a threshold for effects of chrysotile both in in vitro studies and inhalation experiments. The different patterns of gene expression have been confirmed in lungs of rats exposed by inhalation to these types of asbestos. Experiments also suggest no observed adverse effect levels after evaluation of lung injury, inflammation, and fibrosis at lower concentrations of both types of asbestos.