We examine here the in vitro requirements to activate immunocompetent T cells, present among thymocytes, to give rise to CTL, CD4+ T cells producing IL-2 and CD8+ T cells producing IFN-γ. These thymocytes are naive in not having received antigen-dependent signals characteristic of the periphery. Their activation, upon stimulation with allogeneic spleen cells depleted of T cells, referred to here as allogeneic antigen-presenting cells (APCs), to produce allo-MHC-specific effector T cells, requires activated (radiation resistant) CD4+ T cells, syngeneic with the responding thymocytes. We refer here to these T cells as ‘help’. Furthermore, optimal T cell activation requires an Ig+ B220+ cell in the allogeneic APC population, most probably a B cell. The allogeneic APCs cannot be replaced by conventional bone marrow (BM)-derived dendritic cells (DCs) activated by CD40 ligation or exposure to LPS. The requirements for both help and allogeneic B cells in the activation of thymocytes contrast with the requirements to generate substantial responses from splenic T cell populations. Activated, BM-derived DCs stimulate substantial splenic responses without help. These different requirements for activation could reflect the fact that thymocytes have not received an exit-thymus signal and/or that splenic T cells are heterogeneous, containing naive, memory and partially-activated T cells.