The more than 10-year experiences with complement examinations in rheumatic diseases are described: Manifold determinations of the complement in the course of the disease may be valuable diagnostic means, taking into consideration acknowledged criteria of the diagnosis in forms of rheumatic diseases with increasing immune pathogenesis. Particularly important are repeated determinations in the synovial fluid with references to a rheumatoid arthritis and in the serum in systemic lupus erythematodes as well as for the limitation of the earliest manifestations of a systemic sclerodermia with positive proof of antinuclear factors.