Activation of resting (G0) rabbit peripheral blood lymphocytes (PBLs) into DNA synthesis and IgG synthesis was studied using sheep anti-rabbit IgG (SARIgG), protein A, pokeweed mitogen (PWM), and lipopolysaccharide (LPS). DNA synthesis was assayed by [125I]iododeoxyuridine incorporation. IgG synthesis was measured by determination of Ig in culture supernatants by an ELISA assay. Rabbit PBLs cultured with SARIgG or protein A for 48 hr and then without these reagents for 72 hr showed both DNA synthesis and Ig synthesis, whereas PWM and LPS had very little, if any, effect. PBLs stimulated with SARIgG for 6 hr and then without SARIgG for subsequent 114 hr did not become activated into DNA synthesis or IgG synthesis. However, PBLs prestimulated with SARIgG for 6 hr and then with PWM for 114 hr showed prominent DNA and IgG synthesis. LPS also maintained activation of PBLs after prestimulation of these cells with SARIgG, but the effect was much smaller than that of PWM. No evidence was found for production of factors by SARIgG-stimulated PBLs that could, by themselves, either stimulate resting cells or maintain activation of SARIgG-prestimulated cells. These results suggest that anti-IgG and protein A are complete activating mitogens for resting rabbit B cells to proliferate and differentiate into IgG-producing cells, whereas PWM and LPS are not able to activate G0 cells directly, but have a sustaining effect after activation of resting B cells with anti-IgG, either directly or via production of factors by accessory cells.