Synovial fluids and paired sera taken from patients either before, after or at the time of diagnosis of definite rheumatoid arthritis (RA) were compared with samples from patients with unclassified inflammatory arthropathies (IA). Raised levels of immune complexes (IC) were detected in some RA patients by C1q binding activity but in the majority of both RA and IA patients by the platelet aggregation test; levels were usually higher in joint fluids than in sera. IgM rheumatoid factors (RF) and IgA RFs were lower in synovial fluids but IgF RF levels were similar in matched samples. Synovial fluid to serum albumin ratios were used to estimate synovial permeability (inflammation) and then to calculate which patients synthesized macromolecules locally in the synovium. Local synthesis of RFs was detected in a greater proportion of RA than IA patients and only two patients formed RFs locally in the first months of symptoms. Half the patients in both groups however appeared to synthesize or trap IC constituents and in many patients there was evidence of local synthesis within 6 months after their symptoms had started. We conclude that local synthesis of large amounts of RFs is uncommon in the early stages of RA but that IC of unknown composition are synthesized or localized in the affected joints of many patients with RA and inflammatory arthropathies shortly after their symptoms appear.