Patterns of plasma tumour necrosis factor (TNF)-α bioactivity in rheumatoid arthritis (RA) patients treated with infliximab. Using this bioassay three patterns were observed and linked to the clinical response in the 20 patients with a 1-year follow up. (a) The first pattern showed good ability of the plasma to induce IL-6 production before infliximab therapy, followed by complete inhibition 4 hours after the first infusion. Plasma samples before the ninth infusion had low IL-6-inducing activity. This pattern was associated with an American Colege of Rheumatology (ACR) 50 response at 54 weeks (n = 6). (b) In the second pattern the effect of the first infusion was similar to that in the first pattern. However, high IL-6 inducing activity was still present before the ninth infusion but remained sensitive to treatment. In this pattern, five patients had an ACR 50 response but one had no response. (c) In the last pattern the first plasma samples had a moderate or no effect on IL-6 production. This activity was not sensitive to infliximab infusion. All patients (n = 8) had a poor clinical response (ACR <20). A link was observed between these patterns and clinical response (χ2 = 16.6; P < 0.001).