(Chemical Equation Presented) A practical synthesis of porphobilinogen based on the biosynthetic mechanism is described. The crossed Mukayiama aldol reaction is the key step creating the central carbon-carbon bond between the two protected forms of 5-aminolevulinic acids. The optimized sequence gives a crystalline, storable precursor, which can be transformed in high yield into porphobilinogen and bioconjugates thereof. The enzymatic hydrolysis of the precursor produces porphobilinogen in quantitative yield.