The electrostatic field in ribonuclease A monoclinic crystal is computed. A calculation of the electrostatic free energy of interaction between one molecule and its neighbours in the crystal lattice is performed in three crystal planes. It is shown that at the pH value of crystallization for the polymorphic form under investigation the total electrostatic free energy in the monoclinic crystal of ribonuclease A already crystallized is equal to zero. However, the electrostatic free energy along the crystal axis has different values. Electrostatic forces play a negative role for the crystal stability along the alpha-axis while along the other axis the electrostatic interaction supports the crystal state. The electrostatic potential distribution of a ribonuclease A monoclinic crystal at pH 6 is plotted and it is demonstrated that these interactions have a different character in each crystallographic plane.