The binding of the non-steroidal anti-inflammatory drug indomethacin to proteins in human cerebrospinal fluid (CSF), drawn during lumbar puncture from 10 patients affected by lumbosciatica, was measured by equilibrium dialysis and spectrofluorimetry. Similar binding studies on human serum albumin solutions (0.5 and 1 g/L) were performed using the same techniques. The mean binding percentage of indomethacin determined by equilibrium dialysis was 40%. The results obtained by both techniques allowed us to conclude that the binding of indomethacin in CSF was essentially due to albumin.