At an invitation from Oliver Lowry (1910–1996), Kalckar moved to the New York Public Health Institute as a research associate. The move was an important part of developing Kalckar's research biochemical abilities as an enzymologist, because as Paul Berg commented, he “developed a whole new approach to being able to use enzymes in a novel way.” The lab was equipped with a Beckman DU ultraviolet spectrophotometer, introduced in 1941 and still relatively rare; Kalckar rapidly used the instrument to develop several novel enzyme assays. In 1947 he published three papers on purine metabolism enzymes, all of which were highly cited (3,200 citations in 2006).