Short-term combination therapy with the costimulatory antagonists CTLA4Ig and anti-CD40 ligand induces prolonged suppression of disease onset in New Zealand Black/New Zealand White F 1 systemic lupus erythematosus-prone mice. To determine the mechanism for this effect, 20-to 22-wk-old New Zealand Black/New Zealand White F 1 mice were treated with six doses each of CTLA4Ig and anti-CD40 ligand Ab over 2 wk. Combination-treated mice, but not mice treated with either agent alone, had prolonged survival and the production of pathogenic IgG anti-dsDNA Ab was suppressed. Twenty weeks after completion of treatment the frequency of activated B cells producing anti-dsDNA Ab was decreased, and the abnormal transition of T cells from the naive to the memory compartment was blocked. Combination treatment partially suppressed class switching and decreased the frequency of somatic mutations in the V H BW-16 gene, which is expressed by pathogenic anti-DNA Abs. Treated mice were still able to respond to the hapten oxazolone when it was given 8 wk after treatment initiation, and they mounted a somatically mutated IgG anti-oxazolone response that was noncross-reactive with dsDNA. Fifty to 60% of previously treated mice, but only 14% of previously untreated mice, responded within 2–3 wk to a second course of therapy given at the onset of fixed proteinuria and remained well for a further 3–4 mo. Although this treatment had no immediate effect on serum anti-dsDNA Abs or on the abnormal T cell activation observed in sick mice, 25% of treated mice lived for> 18 mo compared with 5% of untreated controls. These results suggest that the effect of costimulatory blockade in remission induction must be mediated by a different mechanism than is demonstrated in the disease prevention studies.