Levels of lactoferrin (LF), antithrombin-III (AT-III) and beta2-macroglobulin (MG) in blood of patients with rheumatoid arthritis (RA), reactive arthritis (ReA) and systemic lupus erythematosus (SLE) were examined for assessment their importance in differential diagnostics of the diseases. It was shown that on the average, LF and AT-III levels were increased at RA, while MG level was practically unchanged. At the same time LF level was stable high regardless of RA activity and duration degree, but its concentration was significantly increased also in ReA patients (33% patients). AT-III, on the contrary, depended on process activity and duration, was more specific to RA, than LF, but its sensitivity at RA was not enough high (from 25 to 44% patients with increased levels subject to disease duration and activity). Coefficient LFE/AT-III, obtained by multiplication of these proteins concentration, had the greatest diagnostic value. Its sensitivity at RA is on the average 85%, and specificity at differential diagnostics of RA--80% vs. ReA and 92% vs. SLE. We consider that coefficient LFbetaAT-III can be used as an additional criterion at differential diagnostics at RA early stages, while other specific antibodies cannot be detected yet.