We used naive CD4 cells and in vitro-derived Th1 and Th2 effectors from TCR transgenic mice to investigate the requirements of these subsets for TCR signaling and interactions with accessory molecules. Peptide Ag and immobilized anti-CD3 were used to provide different TCR signals. Anti-CD28 Ab or a panel of class II+ fibroblasts, expressing no accessory molecules or expressing intracellular adhesion molecule-1, B7-1, or both molecules, were used as APC or accessory cells (AC). An efficient naive T cell response required a strong TCR signal (high dose anti-CD3 or peptide) and high levels of multiple synergizing costimulatory signals, while effector cells responded efficiently to anti-CD3 alone. Addition of AC only slightly augmented the effector response. Effectors responded to lower doses of peptide than naive cells. However, when peptide-pulsed APC were used to stimulate effectors, requirements varied with the cytokine measured. The production of IL-4 did not require accessory molecules on APC. IL-2 production required interacting APC to express accessory molecules, but was little augmented by AC not presenting Ag, suggesting a requirement for noncostimulatory interactions. Proliferation of effectors closely paralleled IL-2 production. Production of IFN-gamma was intermediate in dependence on accessory molecules, and production of IL-5 was nearly as dependent as IL-2. These results establish major differences between the induction of naive and effector responses and document differential requirements for the induction of distinct cytokines, indicating that different cytokines may be produced depending on the context of effector restimulation.