An equimolar mixture of silver methanesulfonate and silver trifluoromethanesulfonate has been investigated as a calibration material for exact mass determination in field desorption mass spectrometry. The mass scale was established using only field desorption by double exposure of the calibration material and the sample on a photographic plate. The precision and the accuracy of the method were tested by recording the mass spectra of a variety of peptides of known composition at a resolution of about 4000. The results indicate that the mass scale established is useful for the determination of the exact masses of field desorbed ions over the mass range examined: m / z 294-1147.