We investigated the effect of Z-100, a polysaccharide-rich extract from Mycobacterium tuberculosis strain Aoyama B, on the protection of immunocompromised mice against infection with Pseudomonas aeruginosa. Mice were given cyclophosphamide and then treated with varying doses of Z-100 for 5 days or left untreated. One day after the completion of the treatment, they were challenged with P. aeruginosa and the mortality was scored. A significantly enhanced protection was observed in the treated group and bacterial number in organs was significantly lower compared with non-treated group. Though the activity of phagocytes did not appear to be activated since there was no difference in chemiluminescence response of peritoneal phagocytes between control and Z-100-treated groups, an enhanced accumulation of phagocytes was observed after bacterial challenge in Z-100-treated group. It was suggested that Z-100 is effective in the restoration of impaired resistance of immuno-compromised mice to bacterial infection possibly through a stimulation of phagocyte accumulation into the site of infection.