CD28 provides a critical costimulatory signal in Ag-specific T cell activation. Recent studies have revealed an important role for CD28 in the development of autoimmune diseases. We have examined the role of CD28 in collagen-induced arthritis (CIA) by inducing CIA in CD28-deficient DBA/1 mice. CD28-deficient mice never developed arthritis and showed markedly decreased levels of IgG and IgM anti-type II collagen (CII) Abs. In addition, the CD28+/− mice had similar levels of IgG1 and IgG2a anti-CII Abs, whereas in the CD28-deficient mice the level of IgG1 anti-CII Abs was decreased compared with that of IgG2a. IFN-γ production by lymph node cells in response to CII was also reduced. CD28-deficient mice were either immunized four times with CII in CFA to augment Ag loading or given low doses of IL-12 to enhance Th1 type responses. Both treatments resulted in a very low incidence of CIA development and minimal disease. CD28-deficient mice developed arthritis from injection of lymph node cells from CII-immunized wild-type mice, followed by immunization with CII in CFA. Taken together, these results indicate that costimulation of CD28 cannot be replaced by repeated activation through TCR or other costimulatory molecules. Thus, CD28 plays a critical role in both cellular and humoral immunity against CII and is indispensable for the development of CIA.