1. The effect of antimycin A on photoreactions of isolated chloroplasts in the μM range was shown to depend strongly on chloroplast concentration, pH of reaction and duration of preincubation. 2. Antimycin A was shown to be an uncoupler of photophosphorylation and an inhibitor of electron transport. The latter activity sets in at lower concentrations, but both activities overlap to a considerable extent. 3. The inhibition of electron transport by antimycin A is potentiated by high pH and by the presence of uncouplers. 4. The uncoupling activity of antimycin A was manifested by inhibiting ATP synthesis (noncyclic and cyclic), stimulation of electron flow and stimulation of the dark decay of the proton pump. 5. The two effects of antimycin A can be separated by the addition of serum albumin, which presumably removes part of the inhibitor from the reaction mixture.