Murine collagen-induced arthritis (CIA) is a T cell-mediated disease which is induced by injection of type II collagen. Previous studies have shown that CD4+ cells which express particular V beta TCR genes are involved in the induction of arthritis in this model. In the present report we demonstrate that CD4-, CD8-, TCR gamma delta cells are present in arthritic joints, expanded in peripheral lymphoid tissue of DBA/1 lac J mice with CIA, and respond in vitro to the anti-TCR gamma delta mAb UC7-13D5 (13D5). In order to directly investigate the role of the gamma delta TCR in murine CIA, DBA/1 lac J mice were injected with 13D5 before or 40 days after injection of type II collagen. Our results demonstrate that i.p. injections of 13D5 initiated 1 day before injection of type II collagen significantly delays both the onset and severity of CIA compared with treatment with type II collagen alone. In contrast, anti-TCR gamma delta mAb injection of arthritic mice 40 days after collagen injection resulted in the rapid onset of severe arthritis which was accompanied by increased bone erosion and cell infiltration into inflamed joints compared with arthritic mice injected with either control hamster IgG or F(ab')2 fragments of 13D5. Arthritic mice injected with intact 13D5 rapidly lost weight, suggesting that 13D5 may induce a cytokine-mediated syndrome similar to that observed in mice and humans after the injection of anti-CD3. Flow cytometry analysis of joint cells isolated after collagenase digestion from arthritic mice demonstrated that 13D5 injection induces the accumulation of CD4-, CD8-, PgP-1 (CD44)+ cells within arthritic joints, whereas arthritic joints from mice injected with control hamster IgG contained cells with a CD4+, CD8- phenotype. CD3+ T cell lines which express the gamma delta TCR from inflamed joints of arthritic mice were established and examined for V gamma usage by the polymerase chain reaction. V gamma 2 rearrangements were predominant in both T cell lines established from inflamed synovium as well as freshly isolated synovial cells from arthritic mice, whereas synovial cells from nonarthritic mice did not demonstrate V gamma 2 rearrangements. Taken together, the results described in this report suggest a direct role for gamma delta TCR T cells in the pathogenesis of CIA in DBA/1 lac J mice.