Antibodies to nuclear antigens (ANAs) are frequently found in the serum of patients with connective tissue diseases (CTDs). Particularly systemic lupus erythematosus (SLE), and have been implicated in the immune-complex mediated pathogenesis of these diseases. In this study we have compared the occurrence of precipitating ANAs in paired samples of serum and synovial fluid from patients with different CTDs. Of the 30 patients examined 3 had precipitating ANAs in their serum only, 1 in the synovial fluid only, and 3 had antibodies in both serum and synovial fluid. Precipitating ANAs in synovial fluid were found in 3/6 patients with SLE, 1 patient with RA/Sjogren's syndrome overlap, and one patient with RA/SLE overlap. Of the other 15 patients with RA, 2 had precipitating antibodies only in their serum. Two of the SLE patients had anti-Sm antibody, one in serum only and the other in both serum and synovial fluid. Detection by ELISA of class specific anti-Sm antibodies in serum or synovial fluid paralleled the occurrence of antidenatured DNA antibodies when both specificities occurred together. One SLE patient did show evidence in synovial fluids of elevated concentrations of specific antibody classes to individual antigens; however, elevated levels were more frequently found in serum. Local production of ANAs does not, therefore, appear to be a feature of synovial fluids from SLE patients.