Two fully blocked and one partially blocked photosystem II nuclear mutants have been selected in Zea mays. The fully blocked mutants lack photosystem II activity, variable fluorescence, the light-inducible C-550 signal, the high potential form of cytochrome b-559, and most or all of the low potential form of the cytochrome. The block in these mutants may primarily affect the reducing side of photosystem II, inasmuch as chloroplasts isolated from both mutants exhibit an elevated F695 fluorescence emission peak. The partially blocked mutant exhibits partial photosystem 11 activity and a reduction, but not the total loss of the variable fluorescence yield, the C-550 signal, and the high potential form of cytochrome b-559. Lamellae isolated from the fully blocked mutants are greatly deficient for a major lamellar polypeptide with an apparent molecular weight of 32,000 daltons, whereas lamellae from the partially blocked mutant show the partial loss of this same polypeptide, suggesting that the 32,000 dalton polypeptide is necessary for the proper function of photosystem I.Genetic mutants are useful tools for the study of photosynthesis, as has been amply documented in the classic work with algal photosynthetic mutants (22). However, with the exception of the large collection of well characterized pigment mutants in barley (17, 36), the number of photosynthesis mutants studied in any single species of flowering plant has been relatively small. To date, most higher plant photosynthesis mutants are characterized by gross pigment losses which are often accompanied by major alterations in thylakoid membrane morphology and the loss of many lamellar polypeptides (9,15,36,37). Recently, a large collection of photosynthesis mutants was obtained in Zea mays (28) using elevated levels of Chl fluorescence to screen specifically for mutants blocked in photosynthesis (29). By employing this technique, both fully green and yellow-green nuclear mutants with blocks in electron transport, photophosphorylation, and dark reactions have been identified (26,28).The present study describes the characteristics of three nuclear photosynthesis mutants in maize. Two of these mutants, hcf*-33 and hcf*-19G, are fully blocked in PSII, whereas a third mutant, hcf*-19YG, is partially blocked in PSII. All three mutations affect not only PSII activity, but also cause the loss of one or both redox