With the recent adaptation of matrix-assisted laser desorption/ionization (MALDI) of large biomolecules and polymers to Fourier transform mass spectrometric analysis, it is of interest to explore the possible advantages of using a "screened" electrostatic ion trapping technique. It is demonstrated that the observed rate of change of ion cyclotron resonance frequency with trapping potential is in excellent agreement with theory, being independent of mass and charge, in the mass range from 1185 to just under 17,000 Da. When a screened cubic trap is used in a ca. 7-T magnetic field, the electrical potential within the trap is effectively reduced, and ion cyclotron resonance frequency shifts as a function of trapping potential are decreased by a factor of 25. There is no evidence of significant improvement in mass resolution with this trap design, except when higher trapping potentials are compared. In a 9.7-V screened trap measurement, a bovine insulin spectrum with an average resolution of 10,000 is obtained. Comparable mass resolution under conventional trapping conditions could not be obtained for trapping potentials greater than 1.2 V. It is evident from this study that use of the screened trap technique could offer advantages for MALDI-Fourier transform mass spectrometry.