Antenna characteristics, such as input impedance are considerably deviated from those in free space when another antenna is located in the vicinity of the antenna. This often causes system degradation problems. In this article, effects of a parasitic wire located beneath the slotted ground plane are investigated on the coupling between monopole antennas above the ground plane. The method of moments is applied to the problem and a combined matrix formulation that includes mutual coupling effects between the elements located in both regions is newly introduced. It is shown that a wire beneath the ground plane considerably affects coupling characteristics between two monopoles above the ground plane when the slot resonates.