AbstractLymphoid development differs sharply between the primary and secondary lymphoid organs. In the former, lymphocytes arise from precursors by antigen-independent processes under thymic or bone marrow microenvironmental influences and undergo extensive selective processes before being allowed to leave. In the latter, lymphocytes with receptors relevant to particular antigens undergo a second wave of proliferation and differentiation leading to the emergence of immunocytes with effector functions. Each of the two sets of events are profoundly dependent on cellular interactions. In the primary lymphoid organs, the“action”centres on stromal cell-lymphoid precursor interactions, and artificial systems permitting B cell formation are much more advanced than those for T cell development. For B cells, IL-7 andc-kitligand (KL) are clearly important but so are as yet undefined stromal cell-derived activities. For thymic development, only fragments of the complex 3-week process of T cell formation can be mimickedin vitroand no IL has unequivocally been shown to be critical.Within the secondary lymphoid organs, where lymphocytes react to the antigenic universe, the key to regulation lies in interactions between accessory cells (dendritic cells, macrophages and their various relatives) T cells and B cells. Efforts to squeeze the relevant cytokines into sharp compartments such as activation factors, growth factors and differentiation factors have been largely unsuccessful. While we can say a great deal about key players such as IL-1, IL-2, IL-4, IL-5, IL-6, IL-10, IFN-γand TGF-βwhich have many well defined effects, thre is also great excitement about newer bioactivities, some of which require cell membrane preparations and others contact with actual living cells. Despite redundancies in factor action as well as synergistic effects, the prospects for rapid progress in understanding of these post-antigenic factors are bright.