The cellular metabolism, acute toxicity, mutagenicity, and carcinogenicity of maleic hydrazide have been reviewed. It seems that this chemical is a mutagen and a carcinogen in cell cultures and animals, but no evidence is available on human carcinogenicity regardless of population exposure in manufacturing, agriculture, and the food chain (i.e., potatoes, potato chips). Because of the level of exposure of the general public to this compound, an epidemiologic survey should be conducted to ascertain possible human health effects. Long-term feeding experiments should be conducted in several animal species to establish whether maleic hydrazide is carcinogenic by this route. Biotransformation and pharmacokinetic studies should be undertaken to obtain better understanding of the chemical's metabolism and excretion. Such investigations would firmly establish whether the tolerance for maleic hydrazide should remain unchanged or whether the use of the compound should be more restricted.