Under positive-ion fast atom bombardment (FAB) mass spectrometric conditions, benzalkonium chloride (BAK) afforded intense peaks at m/z 304 and 332, corresponding to the intact cations [M--Cl]+ of C12 and C14 homologues, respectively. The use of benzethonium chloride as an internal standard and thioglycerol as a FAB matrix allowed the direct and specific determination of the BAK content (0.004-0.020%) in commercial hard contact lens solutions through the individual assay of the two alkyl homologues. A linear relationship between the homologue concentration and the peak-area ratio was observed over the concentration range 3-180 micrograms ml-1.