In order to figure out the relationships between structural features and biological activity of the host-specific HC-toxin in maize, structurally related cyclic tetrapeptides, chlamydocin and CYL-2 were isolated, and their biological activities in maize were examined. Biological activities of preparations were determined by root growth inhibition and electrolyte leakage bioassays. Chlamydocin and CYL-2 showed toxicities to maize. However, the toxicities of these compounds were non-specific. Thus, the precise peptide ring structure of HC-toxin apparently does not play an important role in toxicity, while resistance of maize to HC-toxin is based on a precise ring conformation.