The corresponding levels of azlocillin in serum and in synovial fluid in the knee-joint were investigated in patients who had undergone aseptic surgery of the lower limbs. The mean synovial fluid concentrations for azlocillin were determined on the basis of 30 samples. Clinically relevant azlocillin levels of approximately 40 mu g/ml were recorded in synovial fluid 10 minutes after start of a short infusion of 5 gm. These increased until about 90 minutes after commencement of antibiotic administration when the maximum level was attained. Subsequently synovial fluid levels decreased slowly and approximately 170 minutes after commencement of the short infusion the mean for serum and synovial concentrations corresponded. The results confirm that with an i.v. infusion of 5 g azlocillin levels can be attained for 3 hours in the synovial fluid that are above the break-point for this antibiotic of 64 mu g/ml. However, despite these good pharmacokinetic data it should be remembered that experience has shown that surgical reintervention is often necessary in addition in joint infections to achieve ultimate cure.