Occupational exposure to petroleum products (gasoline) and elevated blood levels of MTBE have been demonstrated among gasoline station attendants. While MTBE and its metabolites have been considered environmentally inert, immunologically these materials could be reactive. This study was conducted to assess the immunological reactivity of humans to MTBE. Blood samples from 24 gasoline station attendants and 12 healthy controls were examined for levels of IgG, IgM, IgA, and IgE against MTBE by ELISA. In the gasoline-exposed group 7 out of 24 exhibited optical densities or antibody levels of 3-15-fold (OD 0.6-2.68) of the levels detected in controls (OD < 0.2). The detected antibodies both against MTBE-BSA or MTBE-HSA were of IgG and IgM but not IgA or IgE isotypes. These antibodies at much lower levels (OD of 0.45) were detected in only 1 of the 12 healthy control groups. The specificity of these antibodies was evidenced by absorption of MTBE antibodies in different sera using the same haptenic group bound to a different carrier. These results indicate that immune reactions to MTBE do occur through hapten carrier reactions which, in some individuals, end with specific IgG and IgM production.