Asymmetric nuclear hybrids have been obtained by fusion of cells from a nitrate-reductase deficient mutant of Nicotiana plumbaginifolia (cnx20) and gamma irradiated protoplasts of Atropa belladonna (irradiation doses tested were 10, 30, 50 and 100 krad). The hybrid formation frequency following selection for genotypic complementation in the NR function was in the range of 0.7%-3.7%. Cytogenetic studies demonstrated that all hybrid plants tested possessed multiple (generally tetra- or hexaploid) sets of N. plumbaginifolia (n = 10) chromosomes along with 6-29 Atropa chromosomes (n = 36), some of which were greatly deleted. Besides the cnxA gene (the selection marker), additional material of the irradiated partner was expressed in some of the lines, as shown by analyses of multiple molecular forms of enzymes. Surprisingly, rDNA genes of both parental species were present and amplified in the majority of the hybrids. Whenever studied, the chloroplast DNA in the hybrids was derived from the Nicotiana parent. Regenerants from some lines flowered and were partially fertile. It is concluded that irradiation of cells of the donor parent before fusion can be used to produce highly asymmetric nuclear hybrid plants, although within the dose range tested, the treatment determined the direction of the elimination but not the degree of elimination of the irradiated genome.