Several lysosomal enzymes were determined in 47 and 62 samples of CSF and plasma, respectively, obtained from MS patients. CSF levels of most enzymes considered were significantly lower in patients when compared to those of the controls, whereas the plasma levels vary little and appeared to be influenced by the course of the disease. The most interesting result is the one concerning the beta-D-glucuronidase in the plasma: in relapsing-remitting patients in the still untreated acute phase, the levels remain noticeably diminished in comparison to controls. The data suggest the potential of using this enzyme to monitor the progression of the disease.