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A 20-kilodalton adenosine nucleotide-binding protein (A-protein) extracted from rod outer segments is shown to catalyze the cholera toxin-mediated ADP-ribosylation of GTP-binding protein (G-protein) from the outer segment. Radiolabel from [adenylate-32P] NAD+ was associated specifically with both the alpha-subunit of G-protein and with A-protein in the presence of activated cholera toxin. In the absence of added A-protein, G-protein appears to undergo ADP-ribosylation at a slower rate. In the absence of G-protein, A-protein was found to be labeled following incubation with [adenylate-32P]NAD+ and cholera toxin. In the presence of G-protein, a light-dependent component of A-protein labeling was observed. A-protein is a labile component of rod outer segments and has an affinity for ADP. The findings suggest that A-protein may act as an ADP-ribosyltransferase in the cholera toxin-mediated ADP-ribosylation of G-protein.