The host-specific toxin of Helminthosporium carbonum Ullstrup has a molecular formula approximating C(32)H(50)N(6)O(10). The compound has been crystallized and a crystalline hydrochloride derivative has been produced. The molecular weight, as determined by chromatography on Sephadex G-10, is slightly less than 700. The toxin appears to be a cyclic peptide, since, although it does not react with ninhydrin or dinitrofluorobenzene, it yields, on hydrolysis, compounds which react to these reagents. It is unstable in dilute acids, yielding ninhydrin-reacting products. Complete acid hydrolysis yields alanine, proline, and three other ninhydrin-reacting components. The infrared spectrum of the toxin reveals an ester band in addition to amide absorption. Its ultraviolet spectrum reveals the presence of unsaturation in the molecule. The toxin is relatively unstable and loses its specific toxicity. This loss of activity appears to be associated with loss of nitrogen and with decreased solubility in water.