We have analyzed peripheral blood mononuclear cell preparations from a rat model of combined injury (CI) [whole-body irradiation (500 cGy 60Co) followed by a thermal injury (20% body surface area, dorsal, scald burn)] for the expression of OX8 antigens. Ficoll-separated mononuclear fractions were labeled with monoclonal antibodies MRC OX8, MRC OX19, W3/13 HLK, or W3/25 for flow cytometric analysis. Combined-injury trauma resulted in decreased mononuclear cells to 6% of normal. This effect was due to the rapid decrease in radiosensitive lymphocytes from 83% to 10%. The relative numbers of monocytes increased from a normal 13% to 70% at day 4 after CI. Labeling of cells with OX8 after CI shifted to a population which was significantly larger in volume than normal lymphocytes. At the same time the mean fluorescence intensity of OX8-positive cells was considerably reduced. With the use of a F(ab) fragment of OX8 as a probe, these results could be partially explained as unspecific binding of the whole molecule of OX8 to Fc receptors expressed by activated monocytes. But double-labeling and cell-sorting experiments also revealed the expression of OX8 antigens by a subset of OX8+/OX19- monocytes after CI.