Administration of killed Propionibacterium acnes to experimental animals leads to the development of hypersensitivity to the lethal and cytokine-inducing effects of endotoxin. This sensitizing property of P. acnes is not always expressed by different bacterial preparations. Its expression depends very much on the conditions employed for the cultivation of this microorganism. The present study investigates which culturing conditions result in P. acnes preparations with optimal sensitizing properties. The composition of the medium, the culturing time and temperature as well as the type of cultivation (in minifermentor or stationary culture) were all varied for this purpose. The resulting bacterial preparations were killed at 65 degrees C for 1 h and tested for sensitizing activity. The results show that stationary cultures of P. acnes grown at 37 degrees C for 4 to 5 days in the appropriate medium produce biologically active preparations with satisfactory sensitizing activity.