Photosynthesis of leaf slices from Capsicum annuum L. was inhibited by specific fractions from high-performance liquid chromatographic separations of extracts from those same plants. Two regions were apparent-a rapidly eluting discrete zone of high potency and a second more diffuse zone of lower activity which would have included phaseic acid. Equivalent fractions derived from solvent blanks were devoid of inhibitory activity. Suggestions by T. Sharkey and K. Raschke (Plant Physiol., 1980, 65, 291-7) that our earlier demonstration (Aust. J. Plant Physiol., 1975, 2, 553-67) of inhibitory activity in plant extracts containing phaseic acid was due entirely to solvent residues is not substantiated. Nevertheless, their case against our proposal that phaseic acid acts as an endogenous regulator of photosynthesis is acknowledged. Present findings do, however, confirm the existence of photosynthetic inhibitors of biological origin.