This article proposes an extension that includes an analytical solution for the coagulation coefficient of bipolarly charged particles by an external electric field with the effect of Coulomb force, based on the known analytical solution for the coagulation coefficient of bipolarly charged particles by Coulomb force without the effect of an external electric field. The proposed solution agreed well with previous numerical results (Journal of Electrostatics 48 (2000) 93). (C) 2004 Elsevier Ltd. All rights reserved.