ABC: a quantum reactive scattering program. This article describes a quantum mechanical reactive scattering program for atom-diatom chemical reactions that we have written during the past several years. The program use a coupled-channel hyperspherical coordinate method to solve the Schrödinger equation for the motion of the three nuclei on a single Born-Oppenheimer potential energy surface. It has been tested for all possible deuterium-substituted isotopomers of the H+H 2 , F+H 2 , and Cl+H 2 reactions, and tried and tested potential energy surfaces for these reactions are included within the program as Fortran subroutines.