The fusion of human HeLa cells with tobacco protoplasts has been accomplished with the use of polyethylene glycol. The sequence from heterocellular adherence to heterokaryon formation has been followed with light microscopy and confirmed by autoradiographs of heterokaryons containing unlabeled tobacco nuclei and tritium-labeled HeLa nuclei. The HeLa nucleus retained its integrity in the tobacco cytoplasm up to 6 days after fusion.