Protoplasts were isolated from leaves of shoot cultures of Nicotiana tabacum var. Xanthi and Petunia hybrida, nuclei from Petunia protoplasts and subprotoplasts from the liquid part of tomato pericarp. They were submitted in several combinations to agglutination by polyethyleneglycol (Kao and Michayluk, 1974) and to fusogenic treatment at pH 9 (Keller and Melchers, 1973) in calcium nitrate solution (Binding, 1974a; Schieder, 1974a). The fate of heterospecific symplasms was investigated during subsequent culture in medium NT-A (Nagata-Takebe medium, modification A: Binding, 1975) and in medium V47 (Binding, 1974b). Cell divisions occurred in tobacco + Petunia and Petunia + tomato symplasms which contained only a few tomato chromoplasts. Incompatibility is supposed to be responsible for the failure of divisions in tobacco + tomato symplasms and in Petunia + tomato symplasms containing a large tomato subprotoplast. The advantage of subprotoplasts for hybridization experiments is discussed in comparison to protoplast fusion and organelle transplantation.