An extracellular polysaccharide was purified from culture supernatants of Paenibacillus jamilae CP-7, a gram-positive bacillus that was isolated from compost prepared with olive mill wastewaters. The extracellular polysaccharide was produced under aerobic conditions in a medium containing olive mill wastewaters (80% [vol/vol]). This exopolymer had a low level of acute toxicity when it is administered to BALB/c mice by the intraperitoneal route. Interesting immunomodulatory effects were detected when mice were given 10 mg of exopolysaccharide per kg of body weight; the proliferative responses of splenocytes to B-cell and T-cell mitogens were suppressed, the in vitro levels of production of gamma interferon and granulocyte-macrophage colony-stimulating factor by unstimulated and lipopolysaccharide-stimulated splenocytes were enhanced, and the levels of resistance to the intracellular pathogen Listeria monocytogenes was increased in mice. Also, the exopolysaccharide was able to induce lymphocyte proliferation in vitro. We conclude that P. jamilae produces an exopolysaccharide with interesting immunomodulatory properties.