Excesses of non-Hodgkin's lymphoma have been observed among farmers exposed to phenoxyacetic acid herbicides and, less persuasively, among workers exposed to insecticides. Exposure to organic solvents (particularly chlorinated hydrocarbons) has also been associated with an increased risk of NHL. TCDD (which is a contaminant of phenoxy herbicides), DDT, and chlorinated solvents have all been reported to induce impairment or suppression of cell-mediated immunity. We hypothesize that NHL is caused by common viruses, such as the Epstein-Barr virus, that induce proliferation and immortalization of B-cells, followed by T-cell impairment entailing cell-mediated immunodeficiency. The increased risk of NHL with HIV infection and heart or kidney transplantation, in which immunodeficiency also occurs, is consistent with this hypothesis.