Paraproteins display a characteristic pattern of limited heterogeneity on isoelectric focusing. In a group of 50 patients with various neurological disorders and paraproteinaemia this pattern could be demonstrated in serum and cerebrospinal fluid simultaneously, regardless of the form of nervous system involvement or of the function of the blood-cerebrospinal fluid barrier. Although there were no qualitative differences in the pattern between paraproteins from benign and malignant cases, a distinct quantitative difference was found: Paraproteins from myeloma patients had significantly more subfractions extending over a wider pH range than paraproteins from patients with benign monoclonal gammopathy. Due to the high sensitivity of isoelectric focusing, more paraproteins could be detected in cases where other electrophoretic methods including immunoelectrophoresis were negative. A surprisingly high percentage of patients was found with benign monoclonal gammopathy and vascular lesions of the central nervous system.