CD28/CTLA-4 interactions with their specific B7-ligands (CD80 and CD86) have decisive roles in antigenic and allogenic responses. Recently, experimental transplant studies demonstrated that donor-specific tolerance is achieved by blocking these interactions. The present study analyzes the expression of these co-stimulatory molecules in peripheral blood cells from 74 liver recipients and in 16 liver biopsies, which were classified into acute-rejection (AR, n = 27) and nonacute-rejection (NAR, n = 47) groups, as well as their influence on the in vitro response of in vivo allosensitized cells. The results clearly indicate that in human liver transplant too, B7 and CD28/CTLA-4 expression on B and CD4(+) peripheral lymphocytes respectively, contributes to graft acceptance or rejection, and appears to be of crucial importance in modulating the host alloresponse and specific-CTL generation. In the NAR-group, costimulatory molecule expression remained at basal levels after transplant, whereas in the AR-group these molecules were significantly upregulated on days of AR. CTLA-4 was observed in the infiltrating lymphocytes in most of the biopsies, but CD80 or CD86 were not. Moreover, specific cytotoxicity from the in vivo primed cells was clearly suppressed in the NAR-patients with low co-stimulatory molecule expression, whereas this activity was not modified but rather stimulated in the AR-group. Together, these findings indicate that intervention of CD28/CTLA-4/B7 signaling could be therapeutically useful in clinical transplantation.