The morphological properties and polypeptide composition of inner chloroplast membranes of normal and mutant pea plants (nuclear mutant "Chlorophyll I") were compared. The total fractions of chloroplast membranes from both sources were separated in a stepwise sucrose density gradient resulting in two fractions. The major fraction from mutant plants, unlike that from normal plants, does not contain thylakoids of the grana but is represented by reticular and cellulate structures, in which each cell corresponds to a thylakoid of macrograna and whose size exceeds that of normal 3 - 4-fold. In addition the inner membranes of mutant plant chloroplasts do not contain membrane polypeptides with molecular weight of 32 000 and 35 000.