An alkaloid-blocked mutant of Claviceps purpurea was isolated from an ergotoxine producing strain. It accumulates chanoclavine I and the corresponding aldehyde. The formation of agroclavine is blocked in this particular strain. By feeding with appropriate intermediates (tetracyclic ergolines) the mutant produces the normal alkaloidal spectrum of the parent strain. Surprisingly, after supplementation with agroclavine, ergosine is synthesized.