This paper numerically evaluates the hydrodynamic drag force exerted on two highly porous spheres moving steadily along their centerline (sphere #1 and sphere #2) through a quiescent Newtonian fluid over a Reynolds number ranging from 0.1 to 40. At creeping flow limit, the drag forces exerted on both spheres were identical. At higher Reynolds numbers the drag force on sphere #1 was higher than sphere #2, revealing the shading effects produced by sphere #1 on sphere #2. At dimensionless diameter (beta, =d(f)/2k(0.5), d(f) and k are floc diameter and interior permeability, respectively) > 20, the spheres can be regarded nonporous. At beta < 20, the drag forces dropped. At beta < 2, the drag forces approached "no-spheres" limit. An increased size ratio of two spheres (d(f1)/d(f2)) would increase the drag force on sphere #1 and reduce that on sphere #2. At increasing beta for both spheres, the drag force on sphere #2 was increased because of the more difficult advective flow through its interior, and at the same time the drag was reduced owing to the stronger wake flow produced by the denser sphere #1. The competition between these two effects leads to complicated dependence of drag force on sphere #2 on beta value. These effects were minimal when beta became low. Two identical spheres could move steadily along their centerline. At higher Reynolds number, the two spheres would move closer because of the incorporation of inertia force. For spheres of different diameters, the sphere # 2 would move faster than sphere #1 regardless of their size ratio and beta value. This occurrence yielded efficient coagulation when two porous spheres were moving in-line. (C) 2003 Elsevier Ltd. All rights reserved.