The presence of activated lymphocytes in the blood of transplant recipients is considered to be a marker of an ongoing immune process. In this study, the clinical utility of spontaneous proliferation (SP) of peripheral blood mononuclear cells (PBMC) as a marker of in vivo activation was evaluated by in vitro [3H]thymidine incorporation in 22 patients in the 1st month after liver transplantation. Also immune activation in the graft was monitored by transplant aspiration cytology (TAC-A). In the study period, there were only 2 mild episodes of clinical acute rejection, where SP of PBMC was 1290 and 1541 cpm (vs. 99 cpm averaged in healthy controls). Though SP of PBMC was also increased in systemic infections, a significant positive correlation was observed between SP of PBMC and TAC-A score. In all episodes where TAC-A score was elevated to above 2, SP of PBMC was simultaneously increased to more than 1000 cpm. In 8 patients prophylactically treated with the anti-interleukin-2-receptor antibody, BT563, SP of PBMC was significantly lower compared to 7 patients treated with antithymocyte globulin. It is suggested that SP can be a reliable parameter of the in vivo activation of lymphocytes, which accompanies immune activation in liver graft, and is potentially useful as a sensitive, although rather non-specific, and non-invasive monitoring of intragraft alloresponse.