The antibacterial effect of a soluble pectin polysaccharide, PMII, isolated from the leaves of Plantago major, was examined in inbred NIH/OlaHsd and Fox Chase SCID mice experimentally infected with Streptococcus pneumoniae serotype 6B. Serotype 6B is known to give a more protracted infection when injected intraperitoneally into susceptible mice than more virulent serotypes like type 4. PMII was administered i.p. either once 3 days before challenge or once to thrice from 3 to 48 h after challenge. The number of bacteria in blood and the mouse survival rate were recorded. Pre-challenge administration of PMII and also lipopolysaccharide (LPS), included as a control, gave a dose-dependent protective effect against S. pneumoniae type 6B infection. However, injection of PMII after establishment of the infection in NIH/OlaHsd mice had no effect. The data demonstrate that, firstly, the polysaccharide fraction PMII from P. major protects against pneumococcal infection in mice when administered systemically prechallenge, and secondly that the protective effect is owing to stimulation of the innate and not the adaptive immune system.