Dr. Blanden is internationally recognised for his contributions to our knowledge of the immune processes which occur in resistance to infectious agents and in recovery from infectious diseases. Although he has shown the importance of cell-mediated immune responses in a number of systems, it is his work, during the last 8 years, on ectromelia infection of mice which represents his major contribution. This constitutes the most complete documentation of the role of cellmediated immunity during infection by a natural pathogen. He was a pioneer in the development of a cytotoxic assay for the analysis of the kinetics and specificity of the T lymphocyte response during the course of the infection. With others, he was responsible for demonstrating that genes in the H-2K and H-2D regions of the major histocompatibility complex of the mouse determine whether or not interaction between effector cells and target cells can occur both in vitro and in vivo. These observations have opened up a new area of investigation in immunology. His use of H-2 mutant mice, in defining the specificity of the interaction, has been particularly elegant. In addition to his experimental findings he has also contributed significantly to theoretical considerations in cell-cell interactions in this system.