To monitor soluble HLA class I (sHLA-I) and their size variants after liver transplantation (LTX) plasma samples from 22 LTX patients were studied by sHLA-I ELISA, SDS-PAGE, and densitometry. Samples collected were classified into three groups: Group 1 comprised samples taken during episodes without complications, group 2 during episodes of cholangitis/cholestasis (CC), and group 3 during episodes of acute rejection (AR). Compared to group 1 (0.27 +/- 0.03 SEM microg/ml) mean sHLA-I increments in groups 2 and 3 were with 0.53 +/- 0.05 SEM microg/ml and 0.47 +/- 0.04 SEM microg/ml increased (p < 0.001). The same samples were studied by SDS-PAGE and the 43, 39, and 35 kD sHLA-I variants were quantified densitometrically. In samples of group 1 ratios of 43 vs. 39 kD bands revealed a mean of 2.1 +/- 0.3, whereas in group 2 and 3 these were only 0.8 +/- 0.1 SEM and 0.9 +/- 0.1 SEM, respectively, (p < 0.001). For the relation between 43 and 35 kD variants a reduced ratio of 1.1 +/- 0.2 SEM was confined to group 3 samples (p < 0.001), as groups 1 and 2 had ratios of 13.4 +/- 2.3 SEM and 8.4 +/- 2.9 SEM, respectively. This indicates that elevated sHLA-I levels during CC or AR are mainly caused by increases of 39 and/or 35 kD sized molecules. Therefore, our study demonstrates, that after LTX the contribution of sHLA-I size variants to total sHLA-I amounts changes drastically during immune activation pointing to different mechanisms of sHLA-I release.