A new procedure for synthesis of 1 4 C-labeled tentoxin [ 1 4 C-MePhe[(Z)Δ] 3 -tentoxin], with a high specific activity, is described. Binding experiments with CF 1 or CF 1 - isolated from spinach chloroplast have been carried out using equilibrium dialysis technique. The results show the presence of two classes of binding sites. The association constants of the two major binding sites were derived from non-linear fitting of the binding curves. At 4°C, the first binding site has a value of K a 1 = 8.2 10 5 M - 1 in CF 1 and 8.7 10 5 M - 1 in CF 1 -oe, while the second binding site has lower affinity with K a 2 = 1.5 10 4 M - 1 in CF 1 and 2.3 10 3 M - 1 in CF 1 - .