Protoplasts isolated from rapidly dividing cell suspension cultures of either Nicotiana sylvestris or tumor derived cultures of Crepis capillaris were fused by PEG or liposome treatments to form homokaryons. Analysis of binucleates by Feulgen microspectrophotometry, and autoradiography, has revealed that whereas fusion products of all cell cycle combinations occur, protoplasts of certain cycle phases participate in fusions more frequently than expected, and there is a slight predominance of "like-with-like" cycle combinations. It is argued that this tendency towards specificity of fusion may be explained by cycle related variation in surface charge on protoplasts, and the mechanisms of action of the fusogens used.