This text provides a perspective on the complex aetiology of cancer. The topics are grouped into two parts: Part 1 covers cross-reactions between carcinogens and modification of chemical carcinogenesis by noncarcinogenic agents; Part 2 examines exogenous factors and endogenous biological parameters that modulate chemical carcinogenesis. The chapters in Part 1 discuss: the combination effects between carcinogens (synergism and antagonism); modification of chemical carcinogenesis by inhibitors, promoters and carcinogens; intercellular communication as an interpretive paradigm for the modification of carcinogenesis; mathematical modelling in the evaluation of carcinogen interactions; and a computerized database search and logic system for the assessment of relative carcinogenic potency of chemical mixtures. Chapters in Part 2 discuss: other exogenous as well as endogenous factors that modulate chemical carcinogenesis; immunotoxicity of carcinogens, diet (caloric intake, protein/amino acids, vitamins, minerals and fibre); age of the organism; hormones and modification of the endocrine status; genetic susceptibility; radiations; viruses; stress; and electromagnetic fields. Because of the dominant role that the oncogene concept, signal transduction and tumour-suppressor genes play in the understanding of the mechanism of carcinogenesis, special emphasis is given to the chapter discussing viral-chemical interactions and the molecular biology of viral tumourigenesis.