We present a program- and machine-independent standard for annotating macromolecular structures. Data encoded by this specification may be used for communicating information about structures and for exchanging it between different computer systems. The format consists of a set of ASN.1 objects which are mechanically straightforward to parse, but are also easy for humans to create and understand. It differs from all other related standards in that it specifies how a molecule should be displayed without requiring a custom format for the coordinate data.