The CCP4 (Collaborative Computational Project, number 4) aims to provide first a state-of-the-art suite consisting of a collection of programs plus associated data and subroutine libraries for the determination of macromolecular structure by X-ray crystallography. The programs are from a wide variety of sources but all use agreed standard data file formats. The suite is designed to be flexible, allowing users a number of methods of achieving their aims and so there may be more than one program to cover each function. The package has been ported to all the major platforms under both Unix and VMS and is freely distributed to academics by anonymous FTP from Daresbury Laboratory. It is widely used throughout the world. Secondly the Project has a responsibility to provide support, both in installing, documenting and maintaining the suite, and in educating budding crystallographers in methodological and computing techniques.