The penetration of telithromycin (HMR 3647), a novel ketolide antimicrobial, has been assessed in an open, single-dose study in eight healthy male subjects. Following a single, oral, 600 mg dose the mean ratio of the concentration of telithromycin in blister fluid over plasma was 1.38. Significant blister fluid concentrations were maintained up to 24 h post-dose. These results indicate that telithromycin penetrates well into inflammatory extracellular fluid, achieving high and sustained concentrations in this medium.