An X-ray structure analysis of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis provides structural details of the pigment-binding sites. The photosynthetic pigments are found in rather hydrophobic environments provided by the subunits L and M. In addition to apolar interactions, the bacteriochlorophylls of the primary electron donor (;special pair') and the bacteriopheophytins, but not the accessory bacteriochlorophylls, form hydrogen bonds with amino acid side chains of these protein subunits. The two branches of pigments which originate at the primary electron donor, and which mark possible electron pathways across the photosynthetic membrane, are in different environments and show different hydrogen bonding with the protein: this may help to understand why only one branch of pigments is active in the light-driven electron transfer. The primary electron acceptor, a menaquinone (Q(A)), is in a pocket formed by the M subunit and interacts with it by hydrophobic contacts and hydrogen bonds. Competitive inhibitors of the secondary quinone Q(B) (o-phenanthroline, the herbicide terbutryn) are bound into a pocket provided by the L subunit. Apart from numerous van der Waals interactions they also form hydrogen bonds to the protein.