Current studies on the biologically relevant characteristics of inhalable fibres are described, including the papers presented in this session. The various cell systems used in in vitro tests, i.e., diploid and permanent proliferating and nonproliferating cells, are listed, as are the different endpoints of these tests. It is shown that use of in vitro tests complements the use of animal experimentation. Opinions on the nature of the acute toxic effects of asbestos fibres on macrophages differ; however, the dependence of toxicity on fibre length has been demonstrated in this system. These data show that the effects of mineral fibres in vitro give an indication of their potential fibrogenicity in vivo. Other cell culture systems, reported in papers in this session, include hamster lung fibroblasts, rat pleural mesothelial cells and mesothelioma cells. Experiments in which fibre geometry is altered, e.g., by acid treatment, indicate that it is an important factor in cytotoxicity; the haemolytic effect of fibres, however, appears to depend on their chemical composition. Thus, a combined physical-chemical effect would appear to be involved. In vitro testing has also made possible investigation of immunological and chromosomal changes due to inhalation of asbestos fibres. The practical use of findings made in vitro is also summarized.