Infectious pancreatic necrosis virus (IPNV) did not replicate in rainbow trout macrophages cultured in vitro, but a limited increase in viral titer was observed when total leucocytes were infected with the virus. Macrophages did not respond directly to IPNV, in that IPNV did not trigger the respiratory burst of these cells, even when the macrophages were previously stimulated. Supernatants were generated from leucocytes and macrophages exposed to the virus, using a variety of incubation periods and viral titers, to investigate whether virus stimulation induced the secretion of any immunomodulatory substances from these cells. No effect on macrophage responses was detected when these supernatants were added to new macrophage monolayers. However, IPNV produced a significant reduction of LPS-induced B cell proliferation, in a dose-dependent manner, indicative of at least one immunosuppressive effect induced by this virus.