Carbon fiber composite materials are used in a variety of applications in industries, and machining processes often generate aerosols of these materials in the workplace. Because the potential health effects of these particles are uncertain, we evaluated the toxicity of a series of carbon fiber composites in vitro using rabbit alveolar macrophages and in vivo using direct intratracheal injection into rat lungs. For comparison, we studied two reference materials, Al2O3 and alpha-quartz, which are inert and toxic, respectively. We measured cytotoxicity in vitro for rabbit alveolar macrophages by trypan blue exclusion and by the release of 51Cr from prelabeled macrophages. We also injected the samples intratracheally into specific pathogen-free rats and 1 month later lavaged the lungs to recover airway cells and fluid. In each of the assays, the quartz was the most toxic substance tested, causing marked cytotoxicity for alveolar macrophages and large increases in airway cells and neutrophils in the rat lungs, whereas the Al2O3 was consistently the least toxic. Three of the composite samples consistently showed little toxicity, whereas two were consistently toxic for alveolar macrophages and caused significant accumulations of airway cells and neutrophils in the rat lungs. The results of the in vitro and in vivo studies produced a similar ranking of toxicity for all of the samples. These studies show that in vitro and in vivo testing is a useful approach to the evaluation of the potential biologic effects of new materials in the lungs. Although these carbon fiber composites appear to be much less toxic than quartz, the data suggest that two of the composites are not inert. Exposure limits more strict than those for nuisance dusts should be used in the workplace for these two composites. Longer term inhalation exposure studies using these two composites are warranted in order to better define their effects in the lungs.