Temperature-regulated expression of recombinant proteins in the tac promoter (P-tac) system was investigated. Expression levels of fungal xylanase and cellulase from N. patriciarum in E. coli strains containing the natural lad gene under the control of the P-tac markedly increased with increasing cultivation temperature in the absence of a chemical inducer. The specific activities (units per milligram protein of crude enzyme) of the fungal xylanase and cellulase produced from recombinant E. coli strain pop2136 grown at 42 degrees C were about 4.5 times higher than those of the cells grown at 23 degrees C and were even slightly higher when compared with cells grown in the presence of the inducer isopropyl beta-D-thiogalactopyranoside. The xylanase expression level in the temperature-regulated P-tac system was about 35% of total cellular protein. However, this system can not be applied to E. coli strains containing lacI(q), which confers over production of the lac repressor, for high-level expression of recombinant proteins. In comparison with the lambda P-L system, the P-tac-based xylanase plasmid in E. coli pop2136 gave a considerably higher specific activity of the xylanase than did the best lambda P-L-based construct using the same thermal induction procedure. The high-level expression tof the xylanase using the temperature-regulated P-tac system was also obtained in 10-litre fermentation studies using a fed-batch process. These results unambiguously demonstrated that the temperature-modulated P-tac system can be used for overproduction of some non-toxic recombinant proteins.