This paper describes a program for 3D similarity searching, called CLIP (for Candidate Ligand IdentificationProgram), that uses the Bron-Kerbosch clique detection algorithm to find those structures in a file that havelarge structures in common with a target structure. Structures are characterized by the geometric arrangementof pharmacophore points and the similarity between two structures calculated using modifications of theSimpson and Tanimoto association coefficients. This modification takes into account the fact that a distancetolerance is required to ensure that pairs of interatomic distances can be regarded as equivalent during theclique-construction stage of the matching algorithm. Experiments with HIV assay data demonstrate theeffectiveness and the efficiency of this approach to virtual screening.