The effectiveness of prophylactic antibiotics is dependent upon both the antimicrobial potency of the drug and the time at which it is first administered. Effectiveness is progressively lost when therapy is delayed, and it is generally recognized that drugs administered only 3-4 hours after contamination of a wound are largely without benefit. Another factor potentially influencing effectiveness in the traumatized patient is the considerable variation in the rate in which various antibiotics penetrate into the interstitial fluid compartment of surgical or traumatic wounds. The present study investigated the effect of route and method of administration of ampicillin, gentamicin, clindamycin, tetracycline, and cephalothin on subsequent wound fluid concentrations. Equivalent doses of each antibiotic were administered by either intermittent intravenous bolus (IV Push), continuous IV infusion (IV Cont) or intermittent intramuscular injection (IM). Peripheral blood and wound tissue fluid from previously implanted stainless steel cylinders were sampled sequentially during an 18 to 24 hour period and assayed for concentration of the antibiotic. Each antibiotic had a different pattern of distribution between serum and wound tissue compartments, but in general, the IV Push method showed comparable levels in wound fluid 4-12 times faster than the IV Cont method. After 12 hours, the highest sustained antibiotic concentrations in wound fluid was usually achieved with the IM route. These data suggest that the earliest and most sustained levels of antibiotic in wound tissue fluid can be achieved by a simultaneous IV Push and IM injection of the drug followed by intermittent IM injections in the normotensive patient or by an IV Push followed by IV Cont administration for patients in shock. These techniques are recommended when it is not possible to administer prophylactic antibiotics before bacterial contamination has occurred, such as regularly occurs in the traumatized patient, especially when treatment is delayed.