Studies on the expanding group of hemopoietic regulators have identified several types of situations indicating the polyfunctionality of these regulators. In actions on hemopoietic populations, this polyfunctionality is seen in cross-lineage actions, in proliferative actions on cells at multiple stages within a lineage and, above all, in actions that do not simply control cell proliferation but also aspects of differentiation commitment, maturation and the functional activity of mature cells. More perplexing are the growing lists of actions on non-hemopoietic tissues, seen in extreme form with the leukemia inhibitory factor group of regulators. The bizarre range of actions exhibited by regulators of this group is difficult to explain but may be indicating the unsuspected existence of some novel integrated bioorgan systems.