The concentration of antibiotic in peripheral lymph reflects the tissue levels. Following microsurgical cannulation of the right lymphatic duct in 28 healthy rabbits, either penicillin-G, nafcillin, erythromycin, or cefazolin was given intramuscularly, and lymph and serum samples obtained at frequent intervals. Penicillin or nafcillin was administered to 14 additional rabbits with cellulitis of the right upper extremity. A serum sample drawn between one and two hours after an intramuscular injection, in general, reflects the peak lymph antibiotic concentration and hence the peak tissue concentration of the drug. After two hours the lymph level remains equal to or greater than the simultaneous serum level. The presence of inflammation did not alter the peak antibiotic levels. However, nafcillin required longer for equilibration in the inflamed state than in the uninflamed state, whereas penicillin-G equilibrated more rapidly in the presence of inflammation.