[1] In his early years as a research scientist, MacLeod, with Oswald Avery and Maclyn McCarty, demonstrated DNA is the molecule responsible for bacterial transformation — and in retrospect, the physical basis of the gene.

[1] At the end of World War II, Congress gave the National Institutes of Health authority to make external research awards, thereby creating its extramural programs, which nowadays constitute almost 90 percent of its funding.

With his background in the newly renamed Department of Defense, MacLeod grew into the role of informal advisor to several NIH directors and served on various grant committees, commissions, and task forces.

Thus, MacLeod had entered the third phase of his highly successful career—the first two being a laboratory research scientist and academic department head—with multiple forays into the realm of science policy and international health.

The same year, NIH Director James Shannon asked MacLeod to work with the Southeast Asia Treaty Organization to find ways to address the problem of cholera.

MacLeod, Smadel, Woodward, Soper, and the other scientific advisors to SEATO recommended the establishment of a laboratory in Dacca, East Pakistan (now Dhaka, Bangladesh), that could conduct field research on cholera.

MacLeod was the first person to hold the position of deputy director of OST and remained there until 1966, serving as an advisor to President Lyndon B. Johnson after Kennedy's assassination.