2′,3′-O-(2,4,6-trinitrophenyl)ADP (= TNP-ADP) is a competitive inhibitor of photophosphorylation of A DP. Here, the interaction of TNP-ADP with the H + -translocating chloroplast ATPase and other thylakoid membrane proteins was studied more in detail and compared with the interactions of the parental compounds ADP and 2,4,6-trinitrophenol (= picric acid). The following results were obtained: (1) Preilluminated thylakoids incorporate about 1 nmol [ 14 C]ADP per mg chlorophyll or the same amount of T N P-[14C]ADP in the following dark. If T NP-[I4C]ADP is present in the light, however, up to 4 nmol label is bound per mg chlorophyll whereas the amount of incorporated [ 14 C]ADP is unchanged. [ 14 C]ADP is exclusively bound to CF 1 , yielding a molar ratio of 1 nucleotide per enzyme. Likewise 1 mol of 14 C label per mol enzyme is found in CF, when thylakoids are illuminated in the presence of TNP-[ 14 C]ADP; the residual bound radioactivity is related with other components of the thylakoid membrane. (2) TNP-ADP as well as picrate are electron acceptors of the photosynthetic electron transport chain. Picramate is one main product of photosynthetic reduction of picrate. Hence at least one of the three nitro groups of the trinitrophenyl moiety is reduced to an amino group. However, additional non-identified, reactive intermediates are formed, which can bind to membrane components. About 20% of the bound label is covalently linked to membrane proteins. (3) The patterns of covalently labeled polypeptides are identical whether [ 14 C]picrate or TNP- [ 14 C]ADP is employed, except for α and β subunits of CF 1 which are specifically labeled in the presence of TNP-[ 14 C]ADP. This reaction is protected by (A DP + P i ) > ATP > ADP > P i . Most likely radicalic intermediates of picrate or TNP-ADP photoreduction are responsible for covalent binding. In case of CF, the radicalic intermediate of TNP-ADP reduction may be accomodated by its ADP moiety into a nucleotide binding site. Subsequent covalent linkage via the reactive phenyl ring substituent seems to occur in both, the nucleotide and the P i binding domain.