The current-voltage characteristic of two tunnel junctions in series is evaluated under the conditions of the Coulomb blockade. With use of the quasiclassical description suggested by Mullen et al. a transport equation for this system is written down and solved, which allows us to give an analytical expression for the Coulomb staircase. The tunnel current below the Coulomb threshold is evaluated to the second order of the perturbation theory. © 1991 The American Physical Society.