A rabbit model was used to determine the penetration of four commonly used antibiotics (clindamycin, tobramycin, cephalothin, and oxacillin) into the nucleus pulposus after receiving an 8-hour course of intramuscular antibiotic injections. Clindamycin and tobramycin achieved therapeutic levels in the nucleus pulposus and both were present in greater than 50% of serum levels. Cephalothin was not detected in the nucleus pulposus and penetrated at less than 4% of serum levels at 1 hour after injection. The data were inconclusive regarding oxacillin penetration.