Bioassays are currently used to measure the presence of functionally active cytokines in biological fluids. These assays may be influenced by the presence of other substances, either cytokine specific or not, in such fluids. In the present study, we analyzed whether some currently used disease-modifying antirheumatic drugs (DMARDs) could interfere with the measurements of circulating interleukin-6 (IL-6) bioactivity in the B9 hybridoma assay. When sera from healthy controls and patients treated with various DMARDs, such as azathioprine (AZA), methotrexate (MTX), intramuscular gold, and sulfasalazine (SASP), were tested in the IL-6 bioassay, an inhibitory effect was observed only with sera from patients treated with AZA. Addition of exogenous AZA, 6-mercaptopurine (6-MP), and MTX to the IL-6 bioassay resulted in a dose-dependent inhibition of the B9 cell proliferation induced by IL-6, AZA being most potent on a molar basis. Concentrations of AZA and 6-MP compatible with serum concentrations achieved in RA patients were able to inhibit the bioassay, but this was not the case for MTX. Exogenous SASP and its metabolites did not modify the IL-6-induced B9 cell proliferation. This study shows that circulating AZA (or its metabolites) exert an inhibitory effect in the IL-6 bioassay. This method is therefore not suitable to measure IL-6 concentrations in patients treated with AZA. Interference of drugs must be ruled out when bioassays are used to evaluate cytokine levels in biological fluids.