Pseudomonas aeruginosa (P. aeruginosa) is the major pathogen in nosocomial and life-threatening infections of immunocompromised or critically ill patients. The macrophage-activating lipopeptide-2 (MALP-2) activates the immune system via Toll-like receptors (TLR) 2 and 6 and leads to an accumulation of immune cells in lungs of young adult (8-10 week old) rats after intratracheal application. This is characterized by a high increase of granulocyte numbers in the BAL 24 h after MALP-2 treatment. It was hypothesized that MALP-2 may have a positive effect on the clinical course of an experimental infection. Therefore, rats were treated with MALP-2 at different time points following an infection with P. aeruginosa. The effect of MALP-2 in combination with immunization with inactivated P. aeruginosa was also investigated. Rats (n = 10) were infected intratracheally (i.t.) with 1 x 10(8) CFU P. aeruginosa on day 0. They were treated on day -3, -1, 0 and +1 with 2.5 microg MALP-2 or the vehicle i.t. In additional experiments, rats were immunized on day -21 and -14 with 1 x 10(8) CFU of inactivated P. aeruginosa bacteria and 2.5 microg MALP-2 or vehicle with 1 x 10(8) CFU of inactivated bacteria and isopropanol. The clinical score, rectal temperature and weight of the rats were checked in both treatment and immunization experiments twice a day. On day 2 they were sacrificed, CFU were determined in the left lung, the right lung being used for histology. In the group treated with MALP-2 1 day prior to infection significant effects were seen: The rectal temperature was about 2 degrees C higher in comparison to the controls at 6 h and also 1 day after infection. Both the symptoms of the infection and the weight loss were significantly reduced. In addition, the CFU and the inflammation in the lung tissue were significantly lower. These effects were not observed after treatment on day -3, 0 or +1. The MALP-2 enhanced immunization only resulted in a tendency to clinical improvement. In conclusion, local immunostimulation at the appropriate time can enhance the host defense against bacteria in the lung.