He worked at the Children's Hospital of Michigan for 35 years, where he oversaw a large amount of pediatric research, particularly in the field of hematology.
He returned to Germany in 1929 and began a doctorate in French literature, but six months before he was due to complete his PhD, he decided to study medicine instead.
When Hitler came to power, however, Zuelzer decided to leave Nazi Germany to finish his medical studies at the German University in Prague, graduating in 1935.
[2] He was a temporary chairman of the hospital's pediatric department in 1946 and was appointed director of the Child Research Center of Michigan in 1955.
[2] He and Gene Kaplan were the first to describe ABO hemolytic disease of the newborn, and Zuelzer published one of the first studies to show that acute leukemias in childhood could be cured by chemotherapy.