Seven monoclonal antibodies to the alpha subunit (G alpha) of the frog photoreceptor guanyl nucleotide-binding protein (transducin or G-protein) have been characterized as to their effect on G-protein function, and this has been correlated in the accompanying paper (Deretic, D., and Hamm, H. E. (1987) J. Biol. Chem. 262, 10839-10847) with the antibody-binding sites on G alpha tryptic fragments. Antibodies 4A, 7A, 7B, 7C, and 7D are members of a class of antibodies that block G-protein activation by light and therefore also block activation of the cGMP phosphodiesterase. All these blocking antibodies also block the interaction of G-protein with rhodopsin as measured by the light-scattering "binding signal," and as measured by the stabilization of meta-rhodopsin II by bound G-protein (extra-meta-rhodopsin II). The antibodies (or Fab fragments) also solubilize G alpha beta gamma from the membrane in the dark under isosmotic conditions and thus interfere with G alpha interaction with the membrane. Antibody 4A also blocks the extra-meta-rhodopsin II generated by G-protein-rhodopsin interaction in detergent solubilized membranes. Thus, even in the absence of phospholipids, antibody 4A blocks G-protein-rhodopsin interaction. Therefore, we suggest that the antibodies recognize a region of G alpha involved with binding to rhodopsin. An alternative hypothesis is that this antigenic site is a region of interaction between the alpha and beta gamma subunits, disruption of this interaction leading to removal of both the alpha and beta gamma subunits from the membrane and blocking interaction with rhodopsin. This does not seem to be the case because the antibodies immunoprecipitate the alpha beta gamma complex, and not just the alpha subunit. Other antibodies, 4C and 4H, do not block phosphodiesterase activation, the light-scattering signal, extra-meta-rhodopsin II formation, or interaction with the membrane in the dark and therefore recognize other sites on G alpha.