The inverse grand-canonical Monte Carlo (IGCMC) simulation is applied to calculate the activity coefficient of polar fluid Molecules of the fluid are represented by hard spheres with a point electric dipole moment immersed in the centre The electron polarizability of the fluid is described by the permittivity E, Good agreement between the IGCMC technique and Widom method data is obtained Comparison with the results of the mean spherical approximation shows that this theory underestimate the Coulombic interactions while the agreement with PT is good. The dependence of the activity coefficient on the number density, electric dipole moment, molecular diameter, and temperature is presented The Coulombic contribution to the activity coefficient is separated. (C) 2010 Elsevier B.V. All rights reserved