Lloyd John Old (September 23, 1933 – November 28, 2011) was one of the founders and standard-bearers of the field of cancer immunology. When Old began his career in 1958, tumor immunology was in its infancy. Today, cancer immunotherapies are emerging as a significant advance in cancer therapy.

Old's contributions to research established many of the principles and priorities of modern tumor immunology. In earlier work, he and his colleagues introduced bacillus Calmette–Guérin (BCG) to tumor immunotherapy; discovered the first link between the major histocompatibility complex (MHC) and disease (leukemia); found the unexpected association between Epstein–Barr virus (EBV) and nasopharyngeal carcinoma; discovered Tumor necrosis factors (TNF); defined the concept of cell-surface differentiation antigens with the discovery of TL, Lyt (CD8), and a range of other mouse antigenic systems; discovered p53, independently with two other groups; and identified the tumor immunogenicity of heat shock proteins. Old was the author or coauthor of more than 800 research publications.