The relationship between concentrations in serum and levels in tissue of flurithromycin, a new fluorinated macrolide, was determined in patients undergoing maxillofacial surgery and thoracotomy. All patients received 500 mg of flurithromycin orally every 8 h. Drug levels in serum, bone, soft tissue, lung, and pericardial fluid were determined microbiologically. The total amount of antibiotic per gram of tissue was calculated on the basis of the concentration in the supernatant of the homogenate. From the parallel course between free concentrations in serum and calculated contents in interstitial fluid tissue, it was concluded that the tissues examined were easily accessible by flurithromycin; penetration values measured by the ratio of areas under the curve were 8.3 for lung, 3.6 for bone, and 0.8 for soft tissue. The results of the pharmacokinetic study suggest that accumulation of the drug during repetitive multiple doses is predictable. Mean residence times were 10.2 and 8.3 h in groups 1 and 2, respectively. For bacteriostatic drugs such as macrolides, not only very high but also prolonged concentrations in tissue lead to favorable therapeutic result.