The effect of the adaptation to toluene on the resistance to different antibiotics was investigated in the solvent-resistant strain Pseudomonas putida S12. We followed the process of the solvent adaptation of P. putida S12 by cultivating the strain in the presence of increasing concentrations of toluene and studied the correlation of this gradual adaptation to the resistance towards antibiotics. It was shown that the tolerance to various chemically and structurally unrelated antibiotics, with different targets in the cell, increased during this gradual adaptation. The survival of P. putida S12 in the presence of antibiotics like tetracycline, nigericin, polymyxin B, piperacillin or chloramphenicol increased 30- to and 1000-fold after adaptation to 600 mg/l toluene. However, cells grown in the absence of any solvents lost their adaptation to toluene even when grown in the presence of antibiotics. Results are discussed in terms of the physico-chemical properties of membranes as affected by the observed cis/trans isomerization of unsaturated fatty acids, as well as in terms of the active efflux of molecules from the cytoplasmic membrane.