T cells play a dominant role in the pathogenesis of collagen-induced arthritis (CIA). The effector mechanism involves a delayed-type hypersensitivity reaction in the synovial joint. Here we describe a new test system, named a flare reaction, to monitor directly this inflammatory T cell function. It is shown that type II collagen (CII) injected into the pinna of the ear is a target for CII-reactive inflammatory T cells formed during the early pathogenesis of CIA. The inflammatory swelling of the joint and the CII-injected ear in CIA rats develop in a coordinated manner. This assay greatly facilitates monitoring of in vivo activated inflammatory T cells during the induction of CIA.