The x-ray analysis of the crystal structure of 8-azaguanine monohydrate is reported. Bond lengths are given with standard deviations of 0.02 A. Bond angles are given with standard deviations of 1 to 2 degrees . All the hydrogen atoms in the structure were found. The hydrogen atom in the imidazo ring is on N9. N8 was found to take no part in the hydrogen bonding pattern in the crystal, but all other possible hydrogen bonds are formed. A very short separation (3.25 A) was observed between the planes of successive 8-azaguanine molecules. This distance is characteristic of charge-transfer complexes. A mechanism for the cell poisoning action of 8-azaguanine is suggested. The mechanism is based upon the assumption that there is an interaction of the charge-transfer type between molecules of 8-azaguanine.