Fast atom bombardment mass spectrometry (FAB-MS) studies of the nonapeptide bradykinin sampled from aqueous glycerol illustrate the importance of the matrix to analyte ratio in the FAB experiment. Enhanced mass spectral results (in conventional direct probe FAB analyses) are characterized by increased ionization efficiency of the analyte together with a substantial reduction in the glycerol background signal. Replicate analyses of different bradykinin/glycerol/water mixtures show this improvement to depend upon the glycerol/bradykinin relationship and not the water/bradykinin relationship in sampling 4 nmol of bradykinin from 1% to 56% aqueous glycerol (2 microL initial sample volumes). The optimum desorption ionization occurs when the molar ratio of glycerol to bradykinin is approximately 300:1. A model emphasizing the importance of the analyte surface concentration in the matrix (with water being simply a convenient vehicle for the deposition of minute portions of glycerol) is proposed to account for the spectral enhancement. The FAB-MS enhancement behavior of four other peptides, selected for their differences in hydrophobicity (and hence, surface activity in glycerol), is compared to that of bradykinin.