"Hard metal disease" is a chronic lung disease characterized by the presence of interstitial fibrosis; the role of cobalt in the development of this entity is still debated. Indeed, most cases have been observed in some workplaces (grinding) of the hard metal industry and in the diamond polishing industry whereas no similar cases have been reported from workers exposed to pure cobalt powders in cobalt refineries. This study was designed to assess the in vitro toxicity (LDH release, morphology) of different cobalt-containing dusts toward murine peritoneal and alveolar macrophages. The results clearly demonstrate that, both in terms of dust particle and cobalt concentrations, the reactivity of the tungsten carbide-cobalt mixture is quite different from that of cobalt metal powder. The ground tungsten carbide-cobalt mixture prepared by the hard metal industry is almost as toxic as crystalline silica whereas, when tested separately, tungsten carbide has no effect and pure cobalt metal powder slightly impairs cell viability. The uptake of cobalt by macrophages in the presence of tungsten carbide was found to be increased. These observations may have some practical implications in industrial hygiene; they suggest that the acceptable exposure level to cobalt metal should be different when handled alone or in association with other powders such as tungsten carbide.