We monitored the immune response after liver transplantation in 20 patients by measuring the serum levels of soluble interleukin-2 receptor (sIL-2R), soluble CD8 (sCD8), serum amyloid-A protein (SAA), and tumor necrosis factor-alpha (TNF-alpha). In six patients data were available to extend the follow-up period to one year. In all patients mean sIL-2R levels increased in the first month after liver transplantation, and subsequently decreased to values similar to pre-OLT ones, while SAA mean levels rose in the first week after OLT only in patients with rejection. sCD8 levels did not significantly rise after OLT, and TNF-alpha was undetectable in most cases. During episodes of rejection, rejector patients had significantly higher levels of sIL-2R, sCD8, and SAA than stable (without complications) patients. Conversely, no significant difference between rejectors and patients with other complications existed for any of the markers studied. These results diminish the importance of these serum markers of immune activation as laboratory tools in the differential diagnosis of acute rejection from other complications. However, sIL-2R, SAA, and sCD8 levels correlated with the histological grade of rejection and therefore can be utilized to monitor patients with an established diagnosis of acute rejection.