The production of cold shock proteins (csps) and cold acclimation proteins (caps) was characterized in the psychrotrophic bacterium Pseudomonas putida Q5 and its transconjugant P. putida Q5T which contains the toluene-degradative TOL (pWWO) plasmid, using two-dimensional gel electrophoresis and computing scanning laser densitometry. Similar growth rates for the psychrotrophic bacterium P. putida Q5 and the transconjugant were found at temperatures ranging from 30 to 0 degree C. Sixteen proteins were quantified and compared in P. putida Q5 and P. putida Q5T following a 25 to 5 degrees C cold shock or constant growth at 5 degrees C. During constant growth at 25 degrees C, a decrease in the synthesis of various proteins occurred in the transconjugant. Following cold shock to 5 degrees C or constant growth at 5 degrees C, csps and caps were produced with a greater number occurring in the transconjugant. This may suggest an additional stress response in the transconjugant owing to metabolic load exerted by the TOL plasmid. Growth of P. putida Q5T with toluate produced seven proteins that appeared to be TOL-plasmid mediated and of which some were also designated as caps.