The International Agency for Research on Cancer (IARC) currently lists 44 individual chemical agents, 12 groups or mixtures of chemicals and 13 exposure circumstances as "Group 1 human carcinogens". A comprehensive search of the published literature revealed that nine of the 44 chemical agents classified as "Group I carcinogens" by IARC have been reported to occur in mainstream cigarette smoke. The other 35 have never been reported to occur in cigarette smoke. The nine agents reported are benzene, cadmium, arsenic, nickel, chromium, 2-naphthyl-amine, vinyl chloride, 4-aminobiphenyl and beryllium. The reported yields of each of these nine agents in mainstream smoke varies widely. The range of yields reported for a given compound is influenced by the type of cigarette tested and when the analysis was conducted. In micrograms/cigarette, the ranges that have been reported for each of the nine compounds are: benzene (0.05-104), cadmium (0-6.67), arsenic (0-1.4), nickel (0-0.51), chromium (0.0002-0.5), 2-naphthylamine (0.0002-0.022), vinyl chloride (0.0013-0.0158), 4-aminobiphenyl (0.00019-0.005) and beryllium (0-0.0005). Although some of the variation in reported yields may be due to differences in analytical methodology, several correlations between the yield of a particular chemical in mainstream smoke and certain cigarette characteristics were observed. For example, charcoal filtration was associated with reduced vinyl chloride, and the concentration of sodium nitrate in the tobacco was positively correlated with the mainstream yield of both 2-naphthylamine and 4-aminobiphenyl. Benzene yield in mainstream cigarette smoke was correlated with the amount of tobacco burned and with the 'tar' level. Agronomic factors such as production practices and soil characteristics, and environmental conditions such as rainfall, reportedly influence the accumulation of metals, for example, cadmium, beryllium, chromium, nickel and arsenic, in the leaf. The use of fertilizers low in nitrate and heavy metals would be expected to substantially reduce the yields of most of the "IARC Group 1 carcinogens" reported to occur in mainstream cigarette smoke. Additionally, modifications in cigarette design, for instance, the use of enhanced charcoal filters or heated instead of burned tobacco, would also be expected to reduce the yields of several of these agents.