Characterization of the protease, HtrA, from pathogen Chlamydia trachomatis is presented. The purified recombinant protein was a serine endoprotease, specific for unfolded proteins, and temperature activated above 34°C. Chaperone activity was observed, although this appeared target-dependent. Inactive protease (S247A) was able to chaperone insulin B-chain, irrespective of temperature, but at 30°C only HtrA and not S247A displayed significant chaperone activity for a-lactalbumin. These data demonstrate that chaperone activity may involve functional protease domain and that C. trachomatis HtrA functions as both a chaperone and protease at 37°C. These properties are consistent with the developmental cycle of this obligate intracellular bacterium.