Nuclei were isolated from various types of donor protoplasts and were transferred into receptor protoplasts in numerous combinations. Five percent uptake was achieved under conditions which did not interfere with viability and subsequent culture of receptor protoplasts. Methodological investigations on nuclei uptake were carried out with cereal and tobacco protoplasts. To look for biological proof of integration and replication of transferred nuclear genes, two complementing, chlorophyll-deficient, light-sensitive mutants of tobacco were used as sources of nuclei and receptor protoplasts. Ca. 5.5 × 10(7) receptor protoplasts were cultured following transplantation experiments involving these complementing mutants and about 1.8 × 10(7) of the resulting calli were subjected to selective conditions which discriminate against the parental types. No nuclear hybrids were detected, although in control experiments somatic hybrids were obtained by protoplast fusion. Some explanations for failure of nuclear hybrid formation are discussed together with other possible approaches for selective somatic combination of plant cell genophores.