Rooted cuttings of the pot-grown Chrysanthemum variety “Hortensien Rose” were irradiated with X-rays, fast neutrons, thermal neutrons and electrons. As soon as the plants grew they were topped to stimulated side-shoot formation, often resulting in complete periclinal chimeras (sports). In addition mutation frequency in a given number of plants is enlarged by such a method. Electrons proved to be ineffective, producing only 6–10% mutated plants. The optimum dose X-rays is 1500 Rads. Both fast and thermal neutrons showed a marked higher mutation frequency, the best dose resulting in both cases in c. 28% of mutated plants. The mutation spectra, e.g. the type of mutations induced, showed some variation after the various treatments. But the number of plants irradiated, as well as the number of mutations induced, do not justify the conclusion that a certain treatment results in a specific mutation spectrum, although there was a tendency towards greater variability after neutron irradiation. The mutation spectrum as well as the frequency greatly depends on the genetic constitution of the treated material. Irradiation of the pink-flowering “Hortensien Rose”, with the maximum number of dominant genes for flower colour, resulted in a great number of different flower colours, as was expected, as well as a number of mutations of flower shape and size. Other pink-flowering varieties showed a similar spectrum, although in some cases the frequency was low or even zero. Chrysanthemum varieties with other flower colours showed a lower mutation rate, most of the flower colour mutations being based on a lower number of dominant genes. Finally, the practical importance and ways of application were discussed.