Two mAbs generated against rhodopsin kinase (RK) were characterized for their epitopes. Both antibodies recognize short peptide sequences, overlapping but distinct, close to the carboxyl terminus. Binding of RK to the antibodies is slow. Attempts were made to use the antibodies immobilized on protein A-Sepharose beads to bind and purify the enzyme. Time-dependent inactivation of the enzyme occurred after its binding to the antibodies. Studies using different conditions to maintain the enzyme in the active form during binding or to reactivate the purified inactivated enzyme were unsuccessful.