Successive transfer in synthetic medium of spores and mycelial fragments from original toxin-producing cultures of Helminthosporium sacchari results in attenuated cultures which do not produce the host-specific toxin helminthosporoside. When attenuated cultures are grown on material obtained from the water wash of sugarcane leaves susceptible to this fungus, the production of heminthosporoside resumes. Compounds that activate toxin production in the fungus are present on the leaf surface and presumably arise via plant metabolism. One activator was identified as a novel free amine, serinol (2-amino-1,3-propanediol). It activates toxin production in attenuated cultures at 1 muM. Several experiments described in this report argue against the selection theory for the attenuation of cultures. The biological significance and some possible mechanisms for the activation of toxin biosynthesis are discussed.