The purpose of the study was to determine the role of lymphocyte subsets (Ly) and reactive oxygen metabolites RSM, concerning the activity of BAL cells, in the pathogenesis of lung involvement in 12 patients with systemic sclerosis (SS) and 4 with systemic lupus erythematodes (SLE) in comparison with 10 control subjects. The cellular activity was measured by means of cytofluorometry (CFM) and chemiluminescence (CL). In SS/SLEY CD3+, CD4+, CD4/CD8-ratio, CD25 + T-Ly and luminol-dependent CL are increased (p less than 0.05). Correlations exist between CD3+, CD4+, CD8+ and CD25 + T-Ly and both luminol-dependent CL and neutrophils (p less than 0.01). The results suggest, that increased secretion of RSM by BAL-cells may be caused by local release of lymphokines by these activated T-Ly. Therefore CFM and CL seem to be useful in addition to BAL cell differentiation in characterizing the BAL cell activity in the diagnostic of lung involvement in SS and SLE.