Levels of rheumatoid factor(RF), β(2)-microglobulin(β(2)MG), C-reactive protein(CRP) and lipid peroxide(LP) in the blood and synovial fluid(SF) and erythrocyte sedimentation rate (ESR) of patients with rheumatoid arthritis(RA) and osteoarthritis(OA) were determined by clinical chemistry. These determinants were analyzed to discriminate RA and OA. Differences in average values of determinants in the blood and SF between RA and OA patients were calculated. Significant differences in the values of ESR, S-CRP, F-CRP and β(2)MG between the two groups were observed. In a discriminant analysis of RA and OA patients by an all possible selection procedure(APSP), nine parameters in blood and SF were used. In the analysis, S-RF, S-CRP, F-RF, F-CRP, F-β(2)MG and ESR were selected as the group of the 6 most effective indices, which gave an 18.4% probability of misclassification. When S-RF and F-RF were excluded, and only 7 factors were used for discriminant analysis, the probability of misclassification was 21.2%. Probability of misclassification using measured values with females was less than that using males and females.