Although all commercial forms of asbestos have been demonstrated to be carcinogenic in animals, so far epidemiological data are controversial concerning what asbestos types are the most carcinogenic and fibrogenic in humans. In order to understand the early cellular events induced by fibrous particles, different in vitro studies (hemolysis, release of enzymes by macrophages, assays on cell culture systems) have been carried out in several laboratories; most of these studies have shown that cell and subcellular in vitro responses were different depending on fiber types: chrysotile versus amphiboles. This presentation compares the results of different laboratories with our data obtained by using a model which modifies the chemistry of the fibers by acid treatment. The acid-leached chrysotile and acid-treated amphibole fibers showed different biological responses in several in vitro systems used in comparison to unleached fibers. These differences in the in vitro reactivity were related to the chemical state of the fibers and might explain the differences in their effects in animals after intrapleural injection as assessed by the percentage of mesothelioma, the latency period, the survival time and the degree of pleural fibrosis. The carcinogenic effect of the fibers is discussed in relation of their in vitro inflammatory or cytotoxic responses.