The influence of a macroion contained in a microdroplet on the mean square dipole moment , which manifests counterion fluctuation and correlation, is investigated through Monte Carlo (MC) simulations. This system models the protein-filled water-in-oil microemulsion droplet. The electrostatic interactions among ions are calculated directly by solving the Poisson equation based on the primitive model. obtained from MC is much larger than that calculated by solving the Poisson-Boltzmann equation. The probability of the macroion location is determined and the average macroion location is always off-center. In comparison with of a hard sphere in a microdroplet without counterions, the neutral particle is repelled from the interface due to electrostatic repulsions among ions. When the interface and the macroion possess charges of similar sign, the macroion is pushed further away from the interface. is a function of counterion concentration, macroion size, and macroion charge Z(p). When the counterion concentration is increased, rises and becomes saturated. At low counterion concentrations, rises significantly as the macroion size approaches the droplet size or as \Z(p)\ is increased. When counterion concentrations are high enough, however, is essentially independent of size and charge of the macroion. (C) 2003 American Institute of Physics.