A skin window technique was used to investigate the ability of three of the newer cephalosporins, moxalactam, cefoperazone, and ceftriaxone, to enter the interstitial fluid of normal, healthy volunteers. Intravenous and intramuscular routes of delivery were compared. The method of delivery did not greatly affect diffusibility with moxalactam and ceftriaxone. Intravenous infusion resulted in greater diffusibility with cefoperazone than did intramuscular injection. Cefoperazone demonstrated the lowest diffusibility of the three test drugs. Ceftriaxone, administered at one-half the dose of cefoperazone and moxalactam, demonstrated the greatest ability to diffuse into interstitial fluid despite its high level of binding to plasma protein. The better diffusibility of ceftriaxone may be explained by a persistently high concentration gradient provided by its long serum half-life. Overall, the results support the concept of the concentration gradient as an important determinant of antibiotic activity.