Cytotoxic T lymphocytes recognize peptide fragments of cytoplasmic proteins presented by class I MHC molecules. Recently, these peptides have been shown to be short, usually nonamers, and their binding to particular class I molecules is now well understood. There is also detailed understanding of how proteins are processed to generate antigenic peptides and some knowledge of the T lymphocyte receptors involved. CTL have been shown to be important in controlling virus infections, including those that cause tumours. CTL have also been demonstrated against other tumours, and their activity may lead to the abnormalities of MHC class I molecule expression that are frequently observed on tumours. The overall role of CTL is probably to survey body cells for abnormalities, including those induced by virus infection and mutation, and to destroy potentially harmful cells.