This symposia series, founded in 1976, is devoted to the advancement and dissemination of knowledge in the field in immunology, particularly as it relates to the immune recognition and responses to protein and peptide antigens. Leading investigators are convened every 2 or 3 years for the purpose of consolidating the research on protein and peptide antigens of defined structure and to focus on these findings in the context of contemporary immunology. Each symposium has focussed on a particular aspect of molecular and cellular immunology of proteins and peptides. It is extremely gratifying that, in the last 2-3 years, the scientific community has shown a heightened interest in the study and understanding of protein and peptide antigens. The third symposium was devoted to viral and bacterial antigens. Great advances have been made in recent years in the elucidation and synthesis of protein antigenic sites. These, together with advances in cloning, expression and sequencing of protein genes, have offered new avenues for the preparation of synthetic vaccines for viral, bacterial and other antigens. Such vaccines have been the aspiration of immunologists for over 20 years. The meeting has served to integrate and correlate the current knowledge of these systems with developing trends in immunology and to identify the most promising new directions for future investigations.