Abstract: Clinician and academic. His research led to the development of a vaccine for Rh-negative mothers to protect their unborn babies at risk of developing Rh haemolytic disease. Consultant physician in renal medicine; director of the renal unit, Royal Liverpool Hospital, 1986–96. Bom 12 June 1930 in Liverpool; died 21 May 2004, aged 73. Ronald Finn was a joint winner in 1980 of the prestigious Lasker prize for clinical research. The award recognised him for “brilliantly elucidating” the immunology of pregnancy. He showed how to protect unborn babies when the blood groups of a fetus and mother were ABO-incompatible. Finn's achievement was the penultimate step in understanding erythroblastosis fetalis, the Rh haemolytic disease in which a pregnant woman's body may reject and destroy her own unborn child. John Ferguson and Louis Diamond first defined the disease in the early 1930s in separate work. Although Finn was unaware of it when he began his research in 1958, considerable rivalry had developed over competing theories to explain the nature of Rh haemolytic disease. In a fresh approach to unravelling an immunological conundrum, he found the disease was caused by the transplacental transmission of maternally formed antibody when mother and baby had incompatible blood types. The discovery opened the way for the development of a protective vaccine that now saves countless lives. Finn studied medicine at Liverpool University and trained at Sefton General Hospital and Liverpool Royal Infirmary. He then chose internal medicine as a specialty. His ambition was to become a clinical academic, but first he needed a substantial research project. Luckily, he was in the right place at the right time. Finn's hunt for a project coincided with plans by Cyril Clarke, a pioneer in clinical genetics, and his colleague Philip Sheppard, to find a research student to study the interaction of the ABO blood groups and Rh factors. Clarke and Sheppard were more concerned with questions about the inheritance of closely linked genetic traits than with haemolytic disease as such. They saw the study of the interaction between blood groups and human disease as an application of the reasoning about genetic inheritance they had derived in exploring evolutionary changes in the colouration of the wings of swallowtail butterflies. Finn knew little about genetics and nothing about Rh haemolytic disease in the newborn. His initial chore was to establish the incidence of Rh disease in the Liverpool area. What surprised him was that only one in 20 Rh-incompatible deliveries yielded a baby with erythroblastosis. Rather than focus on the 5% of sick babies and their mothers, he concentrated on trying to understand the 95% who did not succumb to erythroblastosis. In a prospective study, he monitored the blood cells of Rh-negative mothers during pregnancy and after delivery. He established that fetal red blood cells entered the mother's bloodstream usually at delivery and that, when the fetus and mother were ABO incompatible, the mother's immune system destroyed the cells before she could become sensitised to Rh antigen. In 1960 he suggested that if an Rh-negative mother were given anti-Rh antibody soon enough, any fetal red blood cells in her circulation would be inactivated before her immune system could become sensitised to Rh antigen. The idea was tested initially on Rh-negative male volunteers by injecting them with Rh factor. Using crude semen from sensitised Rh-negative donors, Finn showed that sensitisation could be prevented in Rh-negative male volunteers. More extensive experiments with pure anti-Rh antibody, produced by American investigators who were independently pursuing the similar research, confirmed those findings. In 1964, Finn and Clarke proved their point in a carefully designed clinical trial, among high-risk Rh- negative women. John Cawley, professor of haematology at Liverpool, said, “Professor Finn made the seminal contribution to the solving of rhesus haemolytic disease of the newborn. His introduction of therapy with anti-D saved the lives of thousands of babies worldwide.” His wife Joan and a son and daughter survive him.