The acute phase reaction is an unspecific response to inflammatory stimuli characterized by alterations in the concentration of several plasma proteins. It is of great clinical value to monitor the inflammatory state in patients with rheumatoid arthritis. The erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are the assays most widely used to measure the acute phase response, but there are also several other inflammatory markers (e.g., fibrinogen, haptoglobin, alpha 1-acid glycoprotein, alpha 1-antitrypsin, interleukins (IL), serum amyloid component A (SAA)). We have studied the interrelationships between several of these markers (ESR, Haptoglobin, Fibrinogen, CRP, SAA and IL-6) in rheumatoid arthritis patients. There was a good correlation between all acute phase markers in serum (p < .01). We found especially strong correlations between S-CRP and SAA (p < .000001) and between ESR and P-fibrinogen (p = .000004). The strong correlation indicates that P-fibrinogen could be used instead of ESR in monitoring rheumatoid arthritis patients. This would increase the specificity of the examination as ESR may be influenced by several factors other than the inflammatory response. There were no significant correlations between acute phase markers in serum or plasma and clinical index.