The phytopathogenic fungus Helminthosporium victoriae, a causal agent of Victoria blight disease of a susceptible oat, produces a host-specific pathotoxin complex (HV-toxin or victorin) that has a specific effect on susceptible varieties. 1 ,2) Recently, Macko et al. 2) and Gloer. et al. 3 ) have reported structural studies on victorin C, the major component of the host-specific pathotoxin complex, in which its six structural components (five amino acids and glyoxylic acid) were identified and the primary structure of victorin C (la, Fig. 1) was presented.4 ) The chemical properties and structures of some minor toxins have been so far unknown. In an independent study, we have isolated two host-specific toxin-related compounds (HV-toxins H and M) from a culture filtrate of H. victoriae. Five amino acids were isolated from the hydrolysate of toxin M, and its structure was elucidated, including the absolute configurations of some of its asymmetric carbon atoms, using HR-MS, FAB-MS/MS (first-atom bombardment mass spectrometry/mass spectrometry),5) NMR and ORD spectra (I, Fig. 1). FAB-MS/MS techniques using JEOL JMS-HX100 equipment have been extremely effective in the sequence analysis of toxin M, and made possible the structural confirmation of victorin C. The physical properties and the process for structural assignments of toxin M are reported here.