Upon completing his graduate studies, Kalckar received a Rockefeller research fellowship to spend a year at the California Institute of Technology (Caltech). After arriving in the United States in early 1939, Kalckar briefly visited the Cori lab in St. Louis en route to the west coast. The Coris had unsuccessfully attempted to reproduce Kalckar's oxidative phosphorylation work, and Kalckar was able to point out a key element missing in their experiment. During his St. Louis visit Kalckar and Sidney Colowick (1916–1985) became good friends.