Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a multipotent hematopoietic growth factor, which is mainly produced by T-cells and stromal cells. Beside the stimulating effects on mature granulocytes, it induces the expression of HLA class II-antigen on synovial tissue-cells in patients with rheumatoid arthritis. The concentrations of GM-CSF in the plasma of 87 patients with rheumatoid arthritis, 48 patients with spondyloarthropathy, 17 patients with systemic lupus erythematosus (SLE), and 43 healthy control persons were investigated. We used an immunoradiometric assay (IR-MA) with a detection limit of 30 pg/ml to measure the GM-CSF concentrations in plasma. The GM-CSF levels of 29 patients with severe rheumatoid arthritis (366 +/- 61 pg/ml, p less than 0.05), 58 patients with moderate rheumatoid arthritis (376 +/- 44 pg/ml, p less than 0.0001), and of 17 patients with SLE (256 +/- 41 pg/ml, p less than 0.05) were elevated compared to the control group (174 +/- 18 pg/ml). No significant differences in the mean GM-CSF plasma levels between the patients with spondyloarthropathy (190 +/- 32 pg/ml) and the control group were found. GM-CSF concentrations as high as 1300 pg/ml were detected in the synovial fluids of patients with rheumatoid arthritis. GM-CSF concentrations in the plasma of patients with severe rheumatoid arthritis were correlated with the plasma concentrations of the soluble interleukin-2-receptor (sCD25) (R = +0.53).