Several methods have been described for the analysis of doxycycline in serum which require at least 0.5 mL sample. The assay presented here requires small amounts of serum (0.1 mL) and shows good sensitivity. A Ultrabase C-18 column and mobile phase consisting of acetonitrile-bidistilled water (2872) were used, flow rate 1 mL min^–1 and monitored at 350 nm. Linearity proved satisfactory between 0.025 and 2.5 g mL^–1 of serum. This assay for doxycycline showed good precision (coefficient of variation <6%) and allowed quantitation of serum levels as low as 25 ng mL^–1 using 100 L samples.