Intravenous (i.v.) administration of 300 micrograms methylated BSA (mBSA) to C57-Black mice with unilateral antigen-induced arthritis (AIA) of 4-6 weeks duration resulted in a reproducible flare-up of ongoing arthritis without signs of inflammation in the contralateral non-arthritic knee joint. In the present study we investigated characteristics of the flare-up phenomenon using histological grading and 99mtechnetium pertechnetate uptake measurements to detect and quantitate knee joint inflammation. At 6 h after intravenous mBSA administration flare-up of arthritis was demonstrable, showing histological characteristics of acute joint inflammation; later this progressed to chronic stages, but the inflammatory stage did not return to baseline values up to day 9 after i.v. challenge. Varying the dose of i.v. mBSA from 2 to 1000 micrograms showed that 10 micrograms was sufficient to elicit the phenomenon and also that the latter is dose-dependent. Using mice immunized with both mBSA and methylated HGG (mHGG) with either mBSA or mHGG-induced arthritis, we could show that 4-6 weeks after arthritis induction the flare-up phenomenon is antigen specific. Intra-articular injection of as little as 10 ng of mBSA resulted in exacerbation of inflammation in joints with chronic mBSA-induced arthritis, but induced no inflammation in the contralateral non-arthritic knee joints, indicating local hypersensitivity in the former joints. The flare-up phenomenon may be due to local hyper-reactivity of chronically inflamed joints to minimal amounts of circulating antigen entering the joint.