Abstract: Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, dormancy and plant responses to abiotic stresses. Recently, ABA is demonstrated to be produced by wide range of animals from sponges to various human tissues and cells, such as leukocytes, pancreatic cells, mesenchymal stem cells and to stimulate functions through a signaling pathway involving the intracellular cyclic ADP-ribose and Ca2+. Endogenous ABA stimulates immune responses in leukocytes and macrophages, release of insulin in islet cells, expansion of mesenchymal and colon stem cells, inhibiting growth and induction differentiation of cancer cells. Research indicates that ABA is a universal signal and has potential applications in several human diseases. This article summarizes recent advances on the existence, signal pathways and functions of ABA in animal and human cells.