Joint inflammations were induced in mice by cloned MT4+ Lyt-2- T cells specific for methylated bovine serum albumin. This was done either by intra-articular or by i.v. administration of the cloned T cells, together with local injection of the antigen. Local rechallenge with methylated bovine serum albumin several weeks after waning of the joint inflammation caused a flare-up reaction. The inflammations were quantified by a 99mTc-uptake method and examined histologically. The arthritis induced by the cloned T cells showed aspects of a delayed type hypersensitivity reaction characterized by an intense infiltrate which resembles the inflammation in the human rheumatoid joint. The data presented show that joint inflammations can be induced by T cells only and that, after waning, reexposition to the original antigen can induce a flare-up reaction. The data suggest a central role of T cells in the induction and the exacerbations observed in rheumatoid arthritis.