A sensitive and simple high-performance liquid chromatography assay for benzalkonium chloride (BZK) in biological samples was developed. The biological samples, spiked with domiphen used as an internal standard, were purified by solid-phase extraction. The major homologues of BZK in pharmaceutical products (C(12) and C(14)) were eluted at 24 and 36 min using a YMC-Pack CN column (4.6 x 250 mm, 5 microm) with a mobile phase, mixture of acetonitrile and sodium acetate buffer (48:52), and monitored at 254 nm. The most dominant component C(12) was selected as an indicator to quantify BZK, and adjustment was then done based on the proportion of C(12) in the BZK product. Good linearity was obtained in the range of 0.1-3 microg, and the limit of detection was 20 ng as a loaded amount on column. The recoveries of BZK in serum and tissues ranged from 54 to 90%. In a practical case, 0.16 microg/ml of BZK was quantified in serum collected several hours after accidental ingestion. The method is simple, sensitive and reliable for determining BZK levels in practical biological samples.