In 1921, Heidelberger began to work with Donald D. Van Slyke at the Rockefeller Institute's hospital. Van Slyke had been working to understand the functioning of the kidneys, and more specifically the interaction between oxygen and hemoglobin. At this time, oxyhemoglobin (in which an oxygen molecule is attached to the heme component of the hemoglobin protein) could be produced synthetically in quantities too small to be useful, and also lacked much oxygen-carrying capability. Van Slyke and his team asked Heidelberger to address this problem. In this article, Heidelberger describes a method he developed that created greater yields of oxyhemoglobin with nearly all of its oxygen-carrying power intact.