OBJECTIVES:Infliximab is an anti-tumour necrosis factor-alpha (TNF-alpha) mouse-human IgG1/kappa antibody used to treat patients with rheumatoid arthritis (RA) and other inflammatory diseases. Unfortunately, response failure and side-effects due to immunogenicity of the drug are not rare. In this study, we have compared different methods of assessing drug levels and anti-infliximab antibodies (Abs) and analysed the character of these Abs in sera of RA patients treated with infliximab for 1.5-18 months. METHODS:Functional serum infliximab levels and anti-infliximab Abs were measured by fluid-phase RIAs using 125I-labelled ligands in combination with molecular size and affinity chromatography, and immune complex precipitation. RESULTS:Anti-infliximab Abs were predominantly IgG, 36% being IgG4, and half the immune complexes were lambda-light-chain-positive. Ab titres were associated with inhibition of TNF binding to the drug, and low trough levels of infliximab were most frequent in anti-infliximab Ab-positive sera. Cross-binding to two other anti-TNF drugs was not observed. Detection of anti-infliximab Abs by solid-phase RIA using cross-binding of plastic-fixed and soluble infliximab exhibited low sensitivity and the data were inconsistent with results obtained from binding of the Abs to soluble infliximab. CONCLUSIONS:Specific and neutralizing anti-infliximab antibodies develop in RA patients treated with infliximab, and that low trough levels of functional infliximab are associated with the presence of such antibodies. The most sensitive antibody assay involved binding to soluble and intact infliximab. Assessments of bioavailability and immunogenicity of anti-TNF biologicals may be used to optimize dose regimens and prevent prolonged use of inadequate therapy.