Most of the materials evaluated in this volume are poorly soluble solid materials that are deposited in particulate form in the lung where they may be retained for long periods. In this respect, they should be considered as 'particulate toxicants'. Cobalt and cobalt compounds were formerly evaluated as being possibly carcinogenic to humans and the evidence was inadequate. Since that time, new epidemiological studies have been conducted and are evaluated here. No new studies in experimental animals were available for cobalt compounds used in the hard metal industry. Nevertheless, this volume re-evaluates some of the experimental evidence for cobalt that was presented previously. The Working Group questioned the relevance of the routes of administration used in some of the animal carcinogenesis bioassays for the evaluation of carcinogenicity of cobalt metal and cobalt alloys. The bioassays were reviewed again in this volume.