Cyclo-trans-4-L-hydroxyprolyl-L-serine (JBP485) is a dipeptide isolated from Laennec, and Laennec is a hydrolyzate of human placenta. Evidence has indicated that JBP485 exhibits potent anti-hepatitis activity. In this study, we investigated the protective effect and possible mechanisms of action of JBP485 in Concanavalin A (Con A)-induced hepatotoxicity in vitro. Two in vitro models were established. Model I: primary cultured female rat hepatocytes were only incubated with Con A (50 microg/ml); model II: co-culture system of hepatocytes and autologous splenic lymphocytes, both were stimulated with Con A (20 microg/ml). JBP485 (25 microM) was pre-incubated with the two models. Our results showed that JBP485 reduced cellular aspartate aminotransferase (AST), lactate dehydrogenase (LDH) and tumor necrosis factor alpha (TNF-alpha) leakage following the application of Con A in both of the models. Potential protective mechanisms were elucidated by measuring DNA fragmentations, immunocytochemistry and RT-PCR. We showed that DNA fragmentations in hepatocytes were attenuated in the JBP485 pre-incubated groups, and at the same time, immunocytochemistry and RT-PCR indicated that expression levels of caspase-3 protein and mRNA in the JBP485 treated groups were decreased compared with those in the untreated groups. Moreover, intercellular adhesion molecule-1 (ICAM-1) was also down-regulated by this dipeptide. The results indicate that JBP485 exhibits hepatoprotective effect through inhibition of hepatocyte apoptosis and ICAM-1 expression.