The level of the soluble form of histocompatibility class I antigens, associated with beta(2)-microglobulin (sHLA-I) has been determined by an ELISA sandwich method in serum from burned patients (n=42) and healthy volunteers (n=30). The sHLA-I level was insignificantly increased in burn patients at the stage of burn shock (1284+/-324U/ml, mean+/-S.E.M.) and after day 28 postburn (1368+/-258U/ml) compared to volunteers (1150+/-90U/ml). At the same time a decrease of sHLA-I levels between 4 and 14 days (638+/-178U/ml) was determined (P<0.05). Increased levels of sHLA, though not significant, were detected in patients with TBSAB >70% in comparison to patients with TBSAB from 30 to 70% during burn shock (1493+/-528 and 1075+/-339U/ml, respectively). Expression of membranous HLA class I antigens (mHLA-I) in peripheral blood lymphocytes (PBLs) was assayed simultaneously by indirect immunofluorescence. The number of CD3(+), CD4(+), CD8(+), CD25(+), CD71(+) and CD26(+) lymphocytes was also evaluated. The expression of mHLA-I in PBLs was increased significantly in patients with TBSAB <70% at early postburn period. Daily monitoring showed that the relative numbers of CD25(+) and CD71(+) lymphocytes in patients varied greatly within short intervals of time during burn shock. The data obtained suggest that mHLA-I expression can reflect postburn lymphocyte activation. The serum content of sHLA-I does not depend on lymphocyte number or activated lymphocyte number in peripheral blood at burned patients.