Biomarker research has become the predominant theme for study of human dose-host-response relations to environmental chemicals. Increasing interest has been focused on identifying markers for host susceptibility, with mixed results. Efforts to identify markers for host variability in carcinogenic risk, on the basis of theoretical knowledge of carcinogen metabolism, have been disappointing. New work in the area of acquired risk modifiers, such as nutritional status, is theoretically attractive, but results have been limited. Impressive achievements have been made in the area of immunological variability, which may elucidate the molecular basis of as well as provide practical biomarkers for several diseases. The problem of multiple chemical sensitivities, on the other hand, has proved refractory to biomarker research, reflecting inadequate knowledge of the mechanism and inappropriate application of biomarker methods.