CD4+ T helper cells play a critical role in the chronicity of collagen-induced arthritis (CIA) in mice. The present results focus on the involvement of Th1 and Th2 subsets at the initial stage of the experimental disease in two lines of mice selected for high antibody production: HI that is susceptible, and HII that is resistant to CIA. Both lines are known to be H-2q, display an identical full set of V-beta genes, and mount similar antibody responses to both heterologous and autologous CII. The kinetic analysis of local T cell and anti-bovine CII antibody responses was followed by Elispot assays, the production of interferon-gamma (IFN-gamma) and IgG2a being considered indicative of a Th1 profile, and interleukin-5 (IL-5) as well as IgG1-IgE, of a Th2 profile. The number of IL-5 Elispots is constantly higher in susceptible than in resistant mice. The IFN-gamma production is rather low in HI compared to HII, and besides, preferential help is observed for the Th2-dependent IgG1-IgE isotype-producing B cells in HI, while a switch-over toward IgG2a anti-CII isotype is found in HII. These results suggest that a Th1 preeminence at the onset of the anti-CII response is decisive in the resistance to CIA.