The vitamin B12 (cobalamin) binding proteins, transcobalamin and haptocorrin, were measured in serum and synovial fluid of 12 patients with rheumatoid arthritis (RA) and 11 patients with traumatic synovitis (TS). Levels of both the cobalamin-binding proteins of synovial fluid were markedly elevated in RA compared with TS. In serum, the haptocorrin concentrations were significantly increased in RA vis-à-vis TS. In synovial fluid of RA patients, the transcobalamin and haptocorrin concentrations were positively correlated to the concentrations of macrophages and polymorphonuclear granulocytes, respectively. The demonstration of increased synovial fluid transcobalamin/total protein and haptocorrin/total protein ratios in RA vis-à-vis TS provides further evidence of a local production or liberation of the cobalamin-binding proteins in synovial fluid of patients with RA.