The usefulness of exposing caged fish to polluted water to detect chemical carcinogens was demonstrated by the development of neoplasms on black bullheads. Chemical analysis in this case failed to indicate that that mutagenic or carcinogenic chemicals were present, perhaps because of infrequent sampling. Laboratory exposures of black bullheads to sediment extracts (unpublished data) and brown bullhead embryos to organic extracts of the Tuskegee wastewater failed to cause tumors or other lesions that occurred on fish exposed in cages. Additional refinement of techniques may improve the usefulness of laboratory exposures; however, field exposure of caged fish is currently most satisfactory for the conditions and the carcinogen(s) present in the Tuskegee wastewater.