The influence of various members of the growth and lactogenic hormone family on the immune system is reviewed. A general hypothesis is proposed for growth control in higher animals. It is suggested that immune reactions, which are based on lymphocyte proliferation, obey the general rules of growth control in vertebrate animals. Growth and lactogenic hormones (GLH) are required for the development and function of the immune system and are suggested to deliver the first signal that prepares the cell for proliferation, differentiation and function. This signal has already been designated by others as the competence signal which initiates the cell cycle. Second signals are delivered through the antigen receptor, and/or by some other cell surface receptors (adhesion molecules) and always involve cell-to-cell (‘bridging’) and/or cell-to-matrix interaction. This category of signals is designated as stromal or adherence signals. The lymphocyte adhesion molecules that mediate second signals have evolved form organ- and tissue-specific recognition/regulatory molecules. The antigen receptors have been perfected during evolution from self recognition to specific-antigen recognition. Apart from this exquisitely specific mechanism of immune recognition, there is evidence for other less specific means of recognition by adherence molecules that mediate the activation of the immune system during nonspecific injury and also play a role in the elimination of degenerated and neoplastic cells. Signals delivered through adhesion molecules have the power to commit the cell to a given activity which is executed by the delivery of third signals in the form of soluble cytokines, usually, but not always, by the same cell delivering the second signal(s). The combination of these three groups of signals will ultimately determine whether or not the cell will proliferate, differentiate, maintain function or, perhaps, be committed to apoptosis. Therefore, GLH maintain immunocompetence which enables the immune system to respond to specific antigenic and tissue-derived stimuli in a self-regulated fashion. The adrenocorticotropic hormone-adrenal axis antagonizes the immunostimulatory effect of GLH. This basic pattern of lymphocyte regulation is influenced further by additional hormones, neurotransmitters and neuropeptides, mostly by the modulation of signal delivery. The constant interaction of neuroendocrine and internal immunoregulatory mechanisms assures the fine tuning of the immune system, so that it is able to functiomn homeostasis and harmony with the organism.