Milstein, César (1927-2002), was an Argentine and British molecular biologist who helped develop a method for producing monoclonal antibodies. Monoclonal antibodies are laboratory versions of the antibodies produced naturally by the immune system when foreign substances, such as bacteria and viruses, invade the body. For his work, Milstein shared the 1984 Nobel Prize in physiology or medicine with German biochemist Georges Jean Franz Köhler and British-born Danish immunology researcher Niels Kaj Jerne. Milstein's research was fundamental for providing a better understanding of the workings of the human immune system.

Milstein was born in Bahia Blanca, Argentina. He graduated from the University of Buenos Aires in 1952, and earned a doctorate there in 1957. In 1958, he began studying biochemistry at Cambridge University in England under a British Council Fellowship and earned a second Ph.D. degree in 1960.

In Argentina, Milstein headed the molecular biology division at the National Institute of Microbiology in Buenos Aires. However, the government there imposed policies against liberal intellectuals and scientists, and he resigned his position and returned to Cambridge in 1963. There, he joined the scientific staff of the Medical Research Council's Laboratory of Molecular Biology. He spent the rest of his scientific career there, and headed the laboratory's Division of Protein and Nucleic Acid Chemistry from 1983 to 1994.

Milstein's early work was in the study of enzymes, but back at Cambridge his interest shifted to immunology. During the rest of the 1960's and much of the 1970's he studied antibodies. In 1975, Milstein and Köhler developed a laboratory technique for producing monoclonal antibodies. Their technique has been widely adopted, and has enabled scientists to custom-design antibodies to attack specific infectious agents and to produce identical copies of those antibodies in large quantities. Monoclonal antibodies are also used to study immunity and to diagnose and treat disease, thus greatly advancing the fields of vaccine development and cancer treatment.