The administration of prophylactic antibiotics for the prevention of discitis is common in the field of spinal surgery. The purpose of this study was to determine 1) whether cefazolin administered intravenously would result in detectable intradiscal levels; and 2) if so, what the proper timing of the antibiotic bolus should be to achieve optimal intradiscal concentrations. Forty human lumbar discs were obtained during the study period. All subjects received a preoperative bolus infusion of 2 g of cefazolin during a 3-5 minute period. Serum and intradiscal samples were obtained at intervals ranging from 6 to 220 minutes after the antibiotic infusion. The respective intradiscal concentrations of cefazolin were then determined, and the findings were as follows: 1) cefazolin does diffuse into the human disc in detectable concentrations; and 2) a critical time relationship exists (15-80 minutes after a bolus administration of cefazolin) for the optimal level of intradiscal antibiotics to be achieved.