We have isolated cyclohexane-tolerant mutants from Escherichi coli strain JA300, which is cyclohexane-sensitive and n-hexane-tolerant. These mutants were resistant to low levels of ampicillin, chloramphenicol, nalidixic acid, and tetracycline, and were sensitive to a low level of kanamycin. Spontaneous clones resistant to low levels of the antibiotics, isolated from JA300, showed altered levels of organic solvent tolerance. The clones resistant to ampicillin and chloramphenicol had cyclohexane tolerance. Some of the resistant clones had cyclohexane and n-pentane tolerances. On the other hand, some kanamycin resistant clones became sensitive to n-hexane. Therefore, mechanisms to improve tolerance levels toward organic solvents are closely correlated with some antibiotic resistant system to low levels of antiobiotics.