The concentration of lysozyme (LZM) in cerebrospinal fluid (CSF) has been studied in 148 patients to evaluate its possible significance in the differential diagnosis of various diseases affecting the central nervous system (CNS). In the control group only 3 of 45 patients had detectable LZM in their CSF, the highest value being 1.3 mug/ml. The diabetic and epileptic groups did not differ from the control group. Of 8 patients with primary intracranial tumours, 4 had raised CSF-LZM levels. Twenty of 23 uraemic patients had elevated CSF-LZM, the highest value being 3.3 mug/ml. The highest values were found in patients with bacterial meningitis, tuberculous meningitis and leptomeningitis due to Aspergillus. A positive correlation was found between CSF-LZM and protein concentrations. The measurement of LZM may be of value in the diagnosis of inflammatory processes affecting the CNS and in the diagnosis of certain intracranial tumours.