OBJECTIVE To characterize Listeria monocytogenes from tissues of channel catfish for their ability to cause hemolysis and grow intracellularly in mouse macrophages. METHODS 15 isolates from processed fillets and 15 isolates from the brain, spleen, and kidneys. METHODS Serotype and hemolytic activity of L monocytogenes isolates were evaluated, using plate agglutination and CAMP tests, respectively. Invasiveness of L monocytogenes was determined by inoculating each strain or isolate on J774A.1 macrophage cells. Infected cells were incubated for 0 or 3 hours and lysed; then 100 gli of the lysate was plated onto a brain heart infusion agar plate. Colony counts for each strain or isolate were analyzed statistically. RESULTS Of 30 isolates, 19 were serotype 1 and 11 were serotype 4. Mouse J774A.1 macrophages were inoculated with catfish isolates, a wild-type (EGD) or a nonhemolytic strain of L monocytogenes. Seventy-three percent (11/15) of isolates originating from catfish organs and 100% (15/15) of isolates originating from fillets were not significantly different from the wild-type EGD strain. The nonhemolytic L monocytogenes strain used as a negative control failed to replicate. Intracellular growth of all L monocytogenes isolates decreased after an additional 3-hour incubation period with medium containing 50 [microg/ml of gentamicin. CONCLUSIONS Similar to the wild-type EGD strain, most channel catfish L monocytogenes isolates were hemolytic, serotype 1 or 4, and were invasive for mouse J774A.1 macrophages. CONCLUSIONS monocytogenes growth in mouse macrophages may serve as an in vitro model for determining virulence of isolates from food products or environments.