The effect of T lymphocytes on the IgM, IgG3, IgG1, IgG2b, and IgG2a responses of B lymphocytes to the type-2 T-independent antigens, trinitrophenylated (TNP)-Ficoll, and TNP-Levan, was investigated. T cell-bearing nu/+ mice were found to produce substantially higher IgG2 serum anti-TNP antibody than their athymic counterparts, and nu/nu and nu/+ IgG2a titers exhibiting more disparity than nu/nu and nu/+ IgG2b titers. The Igm, IgG3, and IgG1 anti-TNP levels in nu/nu and nu/+ mice were indistinguishable. By cell transfer experiments, it was determined that this variance in nude and heterozygote IgG2 responses could not be explained by B cell differences between the two strains or by suppressive effects on IgG2 production within nu/nu mice. Rather, the difference was shown to be the result of the absence of T cells at the time B cells were responding to antigen. In the absence of T cells, the strength of the nu/nu anti-TNP antibody response was found to be in the following order: IgM > IgG3 > IgG1 > IgG2b > IgG2a, a heirarchy identical with the recently proposed heavy chain gene order. The possibilities that T cells influence IgG2 production via their specific recognition of IgG2-bearing B cells or via signals to increase heavy chain switching of responding B cell clones are discussed.