The lipocalins, a large, diverse, but relatively poorly understood family of small extracellular proteins, are characterized by the ability to bind small hydrophobic molecules, such as retinol, and by their binding to specific cell surface receptors. These general properties suggest such proteins as appropriate transporters transferring biologically hazardous molecules in a safe and controlled manner between cells. Moreover, many lipocalins have been implicated in the regulation of cell homeostasis: apolipoprotein D, quiescience specific protein, purpurin, alpha-1-microglobulin, and NGAL. This combination of direct and indirect evidence suggests that the lipocalin protein family may be involved, in a quite general way, in the mediation of cell regulation and that many presently functionless family members might act in this way.