Both butadiene and styrene are produced in high volumes; their combined annual production is 20 million tons. They occur together in many industrial and commercial products, including latexes, polymers, adhesives, and particularly in styrene-butadiene rubber tires. Styrene was classified in 1987 within IARC criteria as 2B (possibly carcinogenic to humans); 1,3-butadiene was placed in class 2A (probably carcinogenic to humans) at the meeting of a working group convened by IARC in 1991. This volume brings together the latest information about the toxic hazards of the two compounds. The focus of research on styrene seems to be its neurotoxicological effects and its potential carcinogenicity. Interest in 1,3-butadiene is much more recent and appears to be directed primarily towards species-specific differences in its carcinogenicity. Papers included in this volume were selected so that current data obtained by ambient and biological monitoring and on genetic toxicity and carcinogenicity are amply covered for both compounds. Seven main areas of research are covered: exposures to butadiene and styrene; estimation of internal doses received by exposed people; measurements of metabolites and of adducts with DNA and protein as markers of exposure; neurotoxicity and reproductive effects; genetic toxicity; carcinogenicity; and implications for public health. New results from epidemiological studies of subjects occupationally exposed to butadiene and to styrene are reported. Special consideration is given to the need for developing and validating biological markers of exposure to butadiene and to styrene in order to measure biological effects and susceptibility in human populations, so that adverse effects can be detected earlier and action taken to reduce public health risks. Information on the mechanisms of carcinogenic action is reviewed, since differences between species with respect to such mechanisms can have a major bearing on the extrapolation to human of findings of carcinogenicity in animals. These topics, representing the 'state of the art' with respect to scientific knowledge about the toxic manifestations and mechanisms of 1,3-butadiene and styrene are all covered in this volume.