In laboratory inoculation experiments, Rosen rye seedlings were more susceptible, Genesee wheat seedlings were less susceptible, and Trail barley seedlings were not susceptible to Claviceps purpurea. Guttation fluids from these plants were used as media for the growth of germinating spores of C. purpurea. Growth experiments consisted of spores which were germinated in thin agar films on microscope slides. Germ tube growth was determined by counting the tubes that crossed the perimeter of a square in the microscope field, counting the conidia in the square, and computing the number of crosses per conidium. Rye guttation fluid produced the most growth and barley fluid the least. Thus the degree of susceptibility is correlated with the amount of growth of the parasite in vitro.