1. An attempt has been made to exploit the destructive properties of ionising radiations in overcoming the barriers to hybridisation that exist betweenBrassica oleracea, B. campestris andB. nigra. 2. One series of crosses involvingB. oleracea x B. nigra was successful only after irradiating the gametes or stylar tissues prior to crossing. After exposing female gametes, triploid, and after exposing male gametes, diploid progeny were produced. 3. An explanation of these results is sought in terms of a change in the relationship of embryo and maternal or endosperm tissues.