In this article, David Shemin, who is now in retirement, describes how in 1944 he ingested 66 g of 15N-labeled glycine in order to determine the half-life of hemoglobin and other blood proteins. The ramifications of the experiment led to the unravelling of the biosynthesis of porphyrins and the role of glycine and alpha-aminolevulinic acid in heme, vitamin B12 and chlorophyll synthesis.