A series of compounds related to bis-pyridoxal phosphate has been synthesized and used to crosslink deoxyhemoglobin. The yield of crosslinked hemoglobin increased dramatically from about 15% for the di- or triphosphate to about 70% for the tetraphosphate. The site of attachment of the intramolecular crossbridge was found to be from the N-terminal amino group of one beta chain to lysine 82 of the other. Since the distance between these residues is only 11A, the bis-pyridoxal tetraphosphates probably have a "stacked" conformation. The crosslinked hemoglobins bind oxygen cooperatively but with a greatly decreased affinity. The increased ability to unload oxygen together with the stabilization of the tetramer qualifies them as promising cell-free blood substitutes.