In the course of our search for natural ionophores with ruminant growth promoting properties, we have previously reported the structure of antibiotic M1396032), the first tetronic acid containing polyether. We now wish to describe antibiotic M144255, a monocarboxylic acid type ionophore from a strain of Streptomyces griseus. It has a high affinity for divalent cations especially zinc, for which reason it was given the trivial name, zincophorin. The molecular structure and absolute stereochemistry(Fig.1)were determined by X-ray analysis on the mixed zinc-Magnesium salt(see below)1). Although zincophorin has several unusual structural features which distinguish it from divalent cation ionophores, such as the diacidic ionomycin3)or the A23187/X-14885A4)group, it has the same gross structure as griseochelin5). Both antibiotics are produced by strains of S. griseus and the 1H NMR (CDCl3)of zincophorin(Fig.2)is very similar to that of griseochelin. They are therefore stereoisomers or possibly even identical. The publication of griseochelin during the preparation of our full paper on zincophorin prompts the present communication.