A rapid, sensitive and reproducible high-performance liquid chromatographic (HPLC) assay for cefazolin in rat tissues was developed. Tissue samples were homogenized in distilled water, acidified with 8.5% phosphoric acid, and centrifuged. Cefazolin was isolated from the supernatant by solid-phase extraction on C18 cartridges. The eluate containing cefazolin and internal standard, cephalexin, was injected onto a reversed-phase C18 column and eluted with a mobile phase of 23% methanol in 0.02 M sodium phosphate monobasic (pH 5.0) and detected with UV absorbance at 270 nm. Recoveries of cefazolin were 33.7 +/- 2.5%, 45.4 +/- 2.1%, and 42.9 +/- 1.0% from liver, spleen and lung, respectively. The calibration curves for cefazolin were established at 0.5-1500 micrograms/g in spleen, 0.1-250 micrograms/g in liver and 0.1-75 micrograms/g in lung. The assay was reproducible with within-day and between-day variations of 1-2 and 1-4%, respectively. Application of the assay for tissue distribution of cefazolin in liposomal targeting study was demonstrated.