In vivo tests available to determine the toxicity of mineral fibers are too expensive and time-consuming to be regularly employed in the evaluation of the potential health hazard posed by natural and man-made fibers. In vitro procedures, while economical, convenient and capable of ranking "relative toxicity," are uncertain predictors of specific lesions. Thus, it is of interest to compare the results of various standard in vitro tests with the results of in vivo tests. Data are available for intratracheal and intrapleural exposures of animals to amphibole mineral fibers from UICC amosite and a fibrous form of ferroactinolite. This paper presents data from parallel in vitro studies employing these minerals. The methods used were mammalian erythrocyte lysis, Chinese hamster ovary cell clonal cytotoxicity assay, and rabbit alveolar macrophage cytotoxicity assay. The experiments were conducted in triplicate to determine dose effect by mass and by number of fibers with aspect ratios greater than 3. A comparison of relative toxicity was made between the ferroactinolite and amosite. In the erythrocyte system, there was a greater lytic effect per unit of fibers for ferroactinolite than for the UICC amosite.