In spite of only slightly subnormal pigment contents, two plastome mutants of Oenothera (Valpha, Isigma) were practically incapable of photosynthetic CO2 fixation and another one exhibited considerably reduced photosynthesis (IVbeta). While other photosynthetic enzymes were present as far as investigated, ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) activity was very low or missing altogether. As shown by gel electrophoresis, mutant IVbeta contained some, though little, fraction I protein. In the other two mutants fraction I protein could not be detected. Also, neither the small nor the large subunit of ribulose-1,5-biphosphate carboxylase could be found in these mutants. In immunodiffusion experiments with a monospecific antiserum against rye ribulose-1,5-bisphosphate carboxylase, only extracts from wild-type Oenothera produced visible precipitation lines. Still, the presence of very low levels of immunochemically reactive antigen was indicated for all three mutants. The highest level was observed in mutant IVbeta. The behaviour of the mutant extracts suggested that the antigens of mutant and wild type leaves reacting with the antiserum were not identical. All mutants appeared to have a coupled electron transport system as shown by ATP measurements, light scattering and 515 nm absorption changes. Linear electron transport was possible in the mutants. Still, the photoresponse of cytochrome f and fluorescence measurements suggested altered electron transport properties in the mutants. These are interpreted to be secondary lesions of the photosynthetic apparatus caused by primary deficiency in ribulose-1,5-bisphosphate carboxylase activity. From the absence in two mutants (Valpha, Isigna) of the small subunit of ribulose-1,5-bisphosphate carboxylase, which is known to be coded for by nuclear DNA and to be synthesized on cytoplasmic ribosomes, it appears that the genetic system of the plastids is capable of interfering with the genome-controlled synthesis of plastid components.