Inhibition of alkaloid synthesis by elymoclavine was demonstrated in cultures of intact cells and in protoplast suspension of Claviceps strain SD 58. Assays of dimethylallyltryptophan (DMAT) synthetase in rigorously dialyzed cell-free extracts from protoplasts showed that the level of enzyme present was not significantly affected by exposure of the protoplasts to elymoclavine for 4 to 24 hours. These results suggest the operation of feedback inhibition, but probably not end-product repression, in ergot alkaloid biosynthesis in Claviceps strain SD 58.