Culture supernatants of enteric bacteria isolated from the gastrointestinal tracts of two marine fishes were screened for the production of immunosuppressive substances. At least 50% of the 60 bacterial isolates tested were found to produce immunosuppressants that significantly inhibited the induction of cytotoxic T cells (CTL) in vitro. The strains that secreted the highest suppressive activity was identified as Aeromonas caviae. The immunosuppressive substance purified from the ii. caviae culture supernatant was a polysaccharide with a molecular weight of about 70 kDa that contained mannose:glucose (4 : 1). This polysaccharide did not show nonspecific cytotoxicity, and was found to inhibit the proliferation and differentiation of CD8-positive CTL.