By using X-ray diffraction and immunochemical techniques, we have exploited the use of monoclonal antibodies raised against hen egg lysozyme (HEL) to study systematically those factors responsible for the high specificity of antigen-antibody interactions. HEL was chosen for our investigations because its three-dimensional structure and immunochemistry have been well characterized and because naturally occurring sequence variants from different avian species are readily available to test the fine specificity of the antibodies. The X-ray crystal structure of a complex formed between HEL and the Fab D1.3 shows a large complementary surface with close interatomic contacts between antigen and antibody. Thus single amino acid sequence changes in heterologous antigens give antigen-antibody association constants that are several orders of magnitude smaller than that of the homologous antigen. For example, a substitution of His for Glu at position 121 in the antigen is sufficient to diminish significantly the binding between D1.3 and the variant lysozyme. The conformation of HEL when complexed to D1.3 shows no significant difference from that seen in the free molecule, and immunobinding studies with other anti-HEL antibodies suggest that this observation may be generally true for the system of monoclonal antibodies that we have studied.