Allelism of 9 nitrate reductase deficient (NR-) N. plumbaginifolia cell lines was tested by complementation after protoplast fusion. Complementation was recognized by the appearance of somatic hybrid colonies growing on a selective NH4+/NO3- medium which cannot support the growth of NR- lines. All 5 apoenzyme defective (NA) lines were non-complementing and therefore allelic. The apoenzyme and the cofactor defective (NX) lines were complementing, as expected, and gave somatic hybrids with restored nitrate reductase activity. The 4 cofactor defective lines belong to 3 complementation groups (NX1 and NX9; NX21; NX24). Two of these (NX21 and NX24) are of new types, not previously described in flowering plants. In the somatic hybrids restoration of NR activity was accompanied by the restoration of plant regeneration ability.