The observation that pairs of peptides encoded from complementary strands of nucleic acids can bind one another suggested to us that proteins might use similar 'complementary peptide' sequences to facilitate their interactions with other proteins or peptides. Here we review evidence to support this hypothesis. Investigations by us and by others showed that receptors and their ligands may use complementary peptide sequences or their analogs to facilitate binding. In addition, antibodies made against pairs of complementary peptides bind one another in an idiotypic/anti-idiotypic fashion. Taken together, these findings suggest that complementary peptide sequences may be used as recognition units by pairs of proteins which interact.