Monactin is a member of the macrotetrolide complex produced by a range of Streptomyces species. Monactin has not previously been available for intensive investigation. Early literature reported that the related dinactin is a monovalent cation ionophore with high selectivity for ammonium and potassium. Monactin inhibits T-cell proliferation induced by IL-2 and cytokine production at nanomolar levels for IL-2, IL-4, IL-5 and IFN-γ.