Extracellular P1,P4-diadenosine 5'-tetraphosphate (Ap4A) has been implicated as a modulator of cell stress. We have previously demonstrated specific receptors for Ap4A at the surface of cardiac myocytes (Walker et al., 1993a). In addition, we have isolated a monoclonal antibody (mAb TL4) that recognized the Ap4A receptor and inhibited binding of Ap4A to its receptor (Walker & Hilderman, 1993). As part of our effort to characterize the Ap4A receptor building domain, we screened a random phage peptide library with mAb TL4. After affinity purification of specifically bound phage, we isolated 38 individual phage clones. Twenty-eight of these clones bound mAb TL4 in ELISA and dot blot analyses. Twenty-two of the twenty-eight individual clones contained inserts with an RGS tripeptide sequence. Synthetic RGS peptide specifically inhibits the binding of mAb TL4 to its membrane receptor. Furthermore, the RGS peptide also inhibits [3H]Ap4A binding to its receptor. These data are consistent with the RGS peptide mimicking part of the mAb TL4 recognition site on the Ap4A receptor. The The RGS peptide may be used to help characterize the Ap4A receptor binding domain and to help determine the physiological significance of the interaction between Ap4A and its receptor.