To test whether ATP synthesis could occur via a mechanism of rotational catalysis in which the alpha and beta subunits of F1 would rotate with respect to the minor subunits, we have measured the rate of ATp synthesis after binding various masses of antibodies to F1. If the rotation was an essential feature of the mechanism, the rate of ATP synthesis should be inhibited either completely or proportionately to the load carried by F1. Bivalent immunoglobulins (IgG) or monovalent Fab fragments of an anti-alpha monoclonal antibody (7B3) were bound to F1 present in electron-transport particles in a ratio of 2 Fab or 2 IgG per F1. This binding similarly inhibited the rate of ATP synthesis by a maximum of about 50%. When anti-mouse immunoglobulins were added to the F1-7B3 (IgG) complex, no significant change in the rate of inhibition was observed. In conclusion, the rate of ATP synthesis was the same when F1 was loaded with 100 kDa (2 Fab), 300 kDa (2 IgG, 7B3) or 900 kDa (2 IgG + 4 ant-mouse IgG). It is concluded that the rotation of the alpha subunits is extremely unlikely to play an essential role in the mechanism of ATP synthesis.