The release of chlorophyll, chloroplasts, o-diphenols, o-diphenol oxidase activity and carboxylating enzyme activity during the grinding of maize and sugar cane leaf tissue has been correlated with the breakage of different types of cell. Enzymes of the photosynthetic carbon reduction cycle were released in the grinding stage during which the bulk of the mesophyll tissue was disrupted and grana-containing chloroplasts released. Since the largest amount of phenol oxidase activity and of phenols was also released at this stage it is likely that the enzymes were partly inhibited by phenol oxidation products and, therefore, underestimated. PEP carboxylase is released earlier in the grinding process. It is concluded that the photosynthetic carbon reduction cycle enzymes studied are located in mesophyll cell chloroplasts and that the PEP carboxylase resides outside the chloroplasts, either in the cytoplasm of mesophyll cells or in colourless tissue. These results are discussed in relation to current theories regarding the assimilation and shuttling of carbon dioxide in leaves of tropical grasses.