Degradation kinetics for several peptides that bind to the major histocompatibility complex on antigen-presenting cells were determined in both human serum (HS; 25%) and synovial fluid (SF; 25%) from patients with rheumatoid arthritis to test whether therapeutic intervention of rheumatoid arthritis by direct intrasynovial injection is feasible (at least in terms of peptide stability). Controls consisted of enzymatically immature 10% fetal calf serum and peptidase-rich 5% liver homogenate (all diluted with RPMI-1040 tissue culture medium). Peptide half-lives ranged from approximately 4 to greater than 10,000 min, with most peptides showing half-lives of approximately 10-100 min. These studies show that, even though the populations of inflammatory and other cell types in SF and HS are different (and may, therefore, generate different peptidase profiles), the observed peptide stabilities in SF and HS are similar. This finding indicates that the effect of SF on peptide stability is similar to that of HS.