As a medical intern at Ramos Mejía hospital, Leloir decided to dedicate himself to the laboratory research. From that time, Leloir specialized in metabolism of carbon hydrates.
Early in the 1940s, Leloir was approached by Bernardo A. Houssay, of the institution that later became the Instituto de Investigaciones Bioquímicas de la Fundación Campomar, which Leloir directed for 40 years starting from its foundation in 1947.
Leloir's investigations came fruition during the period he served in this laboratory, simultaneously serving as a visiting professor of the Faculty of Natural and Exact Sciences; this work was interrupted by periods pursuing further studies at Cambridge, the Enzyme Research Laboratory in the United States of America and other leading world centers of scientific research.
Leloir's dedication to his research overcame the economic dificulties confronted faced by the Institute. With homemade tools, Leloir dedicated himself to the study of the internal process by which the liver receives glucose and produces glycogen, the material that organisms use to store energy.
At the beginning of 1948, Leloir's team identified the carnucleotide sugars, compounds that play a fundamental part in metabolism of carbon hydrates, a discovery that brought worldwide recognition to the Institute. Leloir was promptly given the Premio de la Sociedad Científica Argentina, on of few to receive such a prize in a country in which he was a foreigner.
I 1970 he received the Nobel Prize in Chemistry. Later, his team dedicated itself to glycoprotein, and determined the cause of galactosemia, a severe form of lactose intolerance.