Oncogene activation, which appears to play a critical role in carcinogenesis, can be conveniently studied in biological samples via immunologic detection of oncoprotein products with monoclonal antibodies. Immunohistochemical and immunocytochemical detection of oncoproteins can be used to differentiate normal cells from cancer cells, including those of cancers of occupational concern such as lung, bladder, and liver. Furthermore, oncoproteins appear to reach the extracellular environment and are detectable with immunoblotting techniques in the urine and serum of cancer patients. Similar techniques applied to the screening of sera of occupational cohorts with carcinogen exposures indicate that oncoproteins can be detected in workers at increased risk of the development of cancer, and thus oncoprotein detection may be a useful molecular epidemiologic biomarker of preclinical response for the surveillance and prevention of occupational cancer.