AbstractAt present, experiments with laboratory organisms and epidemiological studies are the major source of information about the genetic toxicology of environmental agents. Laboratory systems are limited in value by difficulties in the interpretation of negative results, in quantitation, and in extrapolation from experimental effects of chemicals to specific levels of activity in man. Epi‐demiologic methods measure effects in man but are weakened by long latency times, confounding environmental factors, imprecise endpoints, and high background levels, which reduce sensitivity. Several methodological improvements in genetic toxicity testing are needed, including increased resolving power, greater relevance of observations to effects in man, techniques for evaluating interactions of compounds in chemically complex systems, and improvements in quantitative risk assessment. Because most genetically toxic agents ultimately react as electrophilic agents with nucleophilic centers in cellular macro‐molecules, the quantitative analysis of the resulting products may be a useful approach to the evaluation of the risks posed by exposure to specific chemicals. The main nucleophilic centers in biological macromolecules are thiol and thioether sulfurs, nitrogens in amino groups and rings, and oxygen atoms. Using the laws of reaction kinetics of alkylation and the observed kinetics of induced mutagenic effects, it is possible to relate the formation of alkylated products in macromolecules to genetic toxicity. The alkylation of amino acids (eg, histidine and cysteine) in hemoglobin can be measured with sufficient sensitivity and accuracy to use it as a monitor of exposure to alkylating agents. By determining the degree of alkylation of a specific center, it is possible to calculate the internal dose of an agent and, because erythrocyte life‐spans are relatively uniform, the incremental daily exposure of an individual to an alkylating agent. Dosimetry can be equated with radiologic dose so that exposure can be expressed in rad‐equivalents and the effects of specific agents compared quantitatively to biologically well‐characterized doses of