The outer-membrane (OM) permeability of the psychrotrophic bacterium Pseudomonas fluorescens strain MF0 for the beta-lactam mezlocillin is increased at the optimum growth temperature (28 degrees C) compared to low growth temperatures (8 degrees C). In an attempt to explain this phenomenon, OM protein content was studied in cultures grown at both temperatures. No significant difference in proportion or composition was found, suggesting that a change in the structure and function of porins could be responsible for the differential permeability. The major OM protein OprF of two psychrotrophic P. fluorescens strains, MF0 and OE 28.3, was purified from cultures grown at 8 degrees C and 28 degrees C in order to reincorporate them in solvent-free lipid bilayers. From cultures grown at the same temperature, OprF displayed very similar channel-forming properties for both strains. Decreasing the growth temperature induced a threefold reduction of the major conductance values (250-270 pS in 1 M NaCl for 28 degrees C cultures and 80-90 pS in 1 M NaCl for 8 degrees C cultures). The trypsin digestion kinetics showed a very different reactivity for these porins between cultures grown at 8 degrees C and 28 degrees C. This may indicate that the pore structure of OprF is modified depending on the growth temperature, as suggested by its functional behaviour.