Hydrogen sulfide (H 2 S) has long been known as a toxic gas. However, recently accumulated evidence suggests that H 2 S contributes to a variety of physiologic and pathologic processes. Endogenous H 2 S production is regulated by multiple enzymes that are differentially expressed in the cardiovascular, neuronal, immune, renal, respiratory, gastrointestinal, reproductive, liver, and endocrine systems. Alteration of H 2 S metabolism may affect multiple signaling pathways and tissue homeostasis. The growing number of diverse targets for which H 2 S serves as a gasotransmitter has been extensively reviewed elsewhere. In this review, the authors discuss current emerging evidence that H 2 S regulates mesenchymal stem cell and T-cell functions.