The mutant line “albostrians” ofHordeum vulgare L. was investigated by a variety of methods to detect the presence of the chloroplast coupling factor of photophosphorylation (CF1). The plastids in white leaves of the line “albostrians” lack ribosomes and are therefore not able to synthesize proteins. Plastid membranes were isolated from light-grown and dark-grown leaves of the wild-type and of the plastid ribosome-deficient mutant. CF1 could be removed from chloroplast membranes of the wild-type by treatment with EDTA. However, no ATPase activity was found in EDTA extracts of the mutant, and no trace of CF1 could be detected in this extract by reaction with antiserum against CF1 in two-dimensional immunoelectrophoresis. Eight isoenzymes of ATPase were found in the fraction of soluble proteins of green wild-type leaves. No CF1 is detected in this fraction isolated from mutant leaves, but four of the ATPases (not identical with CF1) are reduced in activity or lacking. Visualization of CF1 upon internal membranes of etioplasts of the wild-type was made possible by a special electron microscope procedure. CF1 particles were found in large numbers attached to the pro-thylakoid membranes. CF1 particles could not be observed upon the membranes from plastids prepared from the mutant. Since no trace of CF1 could be detected in the plastids of the mutant by various methods, it is concluded that the absence of CF1 in this mutant is a direct effect of the deficiency in the capacity of the plastid to synthesize proteins.