Investigations with labelled thymidine indicated that the mononuclear cells of the synovial fluid only rarely proliferateunder physiological conditions. Even in cases of rheumatoid arthritis no elevated proliferation rate was found. Cytophotometric investigations on Feulgen-stained nuclei is another way to determine cellular proliferation. To answer the question if the relative content of neutrophilic granulocytes in synovial fluids influences the proliferation of mononuclear cells, the DNA content of these cells was determined after Feulgen-staining with respect to the number of neutrophilic granulocytes. It was observed that nearly all mononuclear round cells exhibit diploid nuclear DNA content. No effect of the intensity of inflammation (as expressed by the content of neutrophilic granulocytes) was observed. From these results it is concluded that mononuclear cell proliferation is not influenced by the intensity of the inflammation. On the other hand it can be assumed that there is no immunological stimulus for an increassed proliferation of the synovial fluid in rheumatoid arthritis.