A clarification is made of the meanings of the terms 'human carcinogen' and 'animal carcinogen' in cancer research, cancer prevention and legislative actions. A chemical proven to have carcinogenic activity in animals should be considered to represent a risk to humans. However, non-experts tend to consider that 'animal carcinogens' produce cancer only in animals and can be neglected with regard to the human situation. The use of these terms should thus be avoided or they should be used as standard terms based on generally accepted and available definitions.