This monograph evaluates the evidence of carcinogenicity of metallic cobalt particles with or without tungsten carbide, to which workers in the hard-metal industry are exposed, and of cobalt sulfate and other soluble cobalt (II) salts. It also evaluates three other particulate compounds: gallium arsenide and indium phosphide, which are used extensively in the microelectronics industry; and vanadium pentoxide, a contaminant at facilities refining and processing vanadium-rich ores, and in various workplaces that use oil-fired boilers and furnaces.