To be able to calculate the contributions of individual amino acids to the electrostatic field of a protein would be of considerable value in designing proteins of enhanced or altered function and stability. Recent studies on the serine protease subtilisin provide direct measurements of the electrostatic potential in the active site of the enzyme produced by two charged amino acids. We have used these results to test a recently developed method for the calculation of electrostatic interactions between two specific sites on a protein. The extent of agreement between the theoretical and experimental results suggests that the continuum solvent model used in the calculations reproduces the essential features of the interaction.