Wistar rats infected with Streptococcus pneumoniae (type III ATCC) rapidly develop an acute form of experimental lobar pneumonia (ELP) with death of 80-90% of the animals by 6 days after the infection. Prophylactic treatment of these animals with the novel immunomodulator Pidotimod, at the dose of 25 mg/kg bw, significantly increased their rate of survival as compared to the control group (50 vs. 90% respectively). Recovery from the infection appeared definitive since all the Pidotimod-treated survivors were alive and in good condition at the end of the observation period (45 days post infection). Prophylactic treatment with higher or lower doses of the drug was ineffective. Therapy with Pidotimod was not effective. This preliminary study suggests that Pidotimod may have contributed to activation of specific and non-specific immune effectors involved in the host response to S. pneumoniae infection.