11 patients with rheumatoid arthritis were treated with intravenous immunoglobulin (IVGG). In 6 patients clinical results were impressive, although lasting responses could be achieved in 3 patients only. This treatment was immunomodulating, since the immunoregulatory T-cell ratio (CD4/CD8) decreased following therapy by reducing CD4-positive cells in-vivo. By use of anti-mu-antibodies as a B-cell specific mitogen, IVGG-treatment was seen to suppress early processes of B cell activation. In parallel to these cellular effects, IVGG led to a reduction in the levels of polyethyleneglycol-precipitated circulating immune complexes as measured by lasernephelometry.