Infectious pancreatic necrosis virus (IPNV), a member of the virus family Birnaviridae, causes an acute, contagious disease in a number of economically important fish species. CHSE-214, a Chinook salmon embryonic cell line, when infected by IPNV showed morphological and biochemical features of apoptosis, including an intense DNA laddering pattern and blebbing of the plasma membrane, followed by formation of apoptotic bodies. The Mcl-1 gene product proved to be a member of the Bcl-2 gene family, and like Bcl-2 had the capacity to promote cell viability. Here, we investigated the pattern of expression of Mcl-1 in CHSE-214 cells infected by IPNV. We found that the Mcl-1 level decreased markedly in cells undergoing apoptosis after IPNV infection. This decrease was rapid during the first 8 h postinfection and preceded cell death. Furthermore, we found that drugs including cycloheximide, genistein and EDTA either prevented the decline in Mcl-1 levels or blocked the intense DNA laddering pattern. Other drugs like serine proteinase inhibitor, 400 microg/ml aprotinin, 400 microg/ml leupeptin and 100 microg/ml tryphostin did not. The virus gene expression pattern was examined by Western blot using antivirion polyclonal antibody and was blocked during treatment with cycloheximide, genistein and EDTA but not by serine proteinase, aprotinin, leupeptin or tryphostin. Together the data showed a striking correlation between virus replication and Mcl-1 expression in CHSE-214 cells, suggesting that the virus gene expression has a possible involvement with Mcl-1 in the regulation of apoptosis in these cells.