Electron microscopy has been used to assess the efficiency of four different techniques as potential methods for fusing isolated plant protoplasts. Protoplast aggregation has been achieved by the use of an antiserum, high pH, polyethylene glycol 6000 or poly-L-lysine. The ultrastructural characteristics of each treatment are described. Fusion of adjacent surface membranes does not necessarily fellow aggregation, and has not been observed in protoplasts aggregated by antisera. The other three methods all result in some degree of fusion, depending upon the state of the protoplasts and the temperature of the experiment. Fusion is favoured by a high density of cytoplasm in the protoplasts and by higher temperatures. The methods are discussed in terms of their likely applicability as a practical fusion technique.