Previous investigations have suggested that pairs of peptides specified by complementary RNA sequences (termed complementary peptides) can interact with one another. In the light of this finding, we hypothesized that an antibody directed against a peptide might interact with an antibody against its complementary peptide at the antigen combining site. To address this possibility, polyclonal antibodies against a peptide, Leu‐Glu‐Arg‐Ile‐Leu‐Leu (LERILL), and its complementary peptide, Glu‐Leu‐Cys‐Asp‐Asp‐Asp (ELCDDD), were made monospecific by affinity chromatography. Using radioimmunsassays, anti‐ELCDDD antibodies were shown to interact with 125I‐anti‐LERILL antibodies but not with 125I‐control antibodies. More importantly, the interaction of the two antibodies could be blocked using either peptide antigen, but not with control peptides. Furthermore, 125I‐anti‐LERILL binding to LERILL could be blocked with anti‐ELCDDD antibody and vice versa. We concluded therefore that antibody/antibody binding occurred at or near the antigen combining site, demonstrating that this interaction was an idiotypic/anti‐idiotypic one. Copyright © 1989 Heyden & Son Ltd.