The reactions of formaldehyde with amino acids and diamines were studied. The rates and, where possible, the equilibria for methylene bridge ring formation were determined. The slow step was one of three possibilities: (1) intermolecular reaction of formaldehyde with amino acid; (2) removal of hydroxyl from the product of this reaction; or (3) ring closure. The rate of the ring closure step varies by more than a billion from phenylalanine to some of the other amino acids. Variation of the ring-chain equilibrium constant among five amino acids correlated well with hydroxymethyl adduct equilibrium constants for model compounds. With histidine, a methylene bridge formed between the Alpha-amine and an imidazole nitrogen, but this slowly gave way to a methylene bridge between the Alpha-amine and C4 of the imidazole. Attempts are being made to grow crystals of serum albumin large enough for x-ray crystallography.