The interactions of non-host plants with sunflower, corn and cowpea rusts were compared with those described previously for other cowpea rust-non-host combinations. Compared with the situation on the respective host plant, all three rusts made fewer attempts at penetration into the leaf on half or more of the non-hosts examined. Nevertheless, in no case could such surface behavior completely account for non-host resistance since at least a few attempts at penetration were observed for all the non-host-rust combinations. In only one non-host, cabbage, was linear growth inside the leaf significantly less than that in the respective susceptible plant during the first 24 h after inoculation. Since, for cowpea and sunflower rusts at least, growth in this non-host was also less than that achieved on artificial membranes, it was concluded that non-host resistance of cabbage leaves involved an inhibition of fungal growth. In contrast, a detailed study using cowpea rust showed that the extent and pattern of growth in most of the other non-hosts was similar, apart from a greater frequency of haustorial mother cells, to that on the membranes. Thus it seemed that these non-hosts had little influence on the development of the fungus except in the induction of the haustorial mother cell septum. Susceptible plants, however, appeared to stimulate the formation of secondary hyphae, and it is postulated that the resistance of many non-hosts to this rust, and others, may be due to the absence of any similar stimulation by the plant. The importance of the haustorium in this stimulation could not be clearly determined. The possibility that the non-host plant may actively inhibit haustorium formation, however, was suggested by the fact that their absence in many non-host-rust combinations could be correlated with the darkening of the plant wall adjacent to the haustorial mother cell. The general lack of specificity of non-host responses to rust fungi was indicated by the observation that where a given non-host-rust combination was characterized by a particular feature, such as the presence of wall darkening or the relatively high frequency of haustoria, then a similar feature was often seen following infection of the same non-host with the other two rusts. The observation of a few instances where this was not the case, however, suggested that the nature of the fungus may have a modifying effect on the plant response.