A significant elevation of serum sialyltransferase and fucosyltransferase mean activities was observed in 19 untreated patients with multiple myeloma. However, sialyltransferase mean activity was significantly lower in 13 other patients treated for 1-30 months with alkylating drugs and prednisolone. Such a definite decrease in serum enzyme activity on treatment was not recorded for fucosyltransferase. Instead, this activity was significantly increased in treated patients as compared to controls. The presenting clinical features of the 32 patients with multiple myeloma were the basis for a clinical staging system with regard to myeloma cell burden according to established criteria. In untreated patients (as opposed to treated ones), a significantly higher serum sialyltransferase (but not fucosyltransferase) activity was obtained among those 11 belonging to stage III than among the other eight with stages I and II, suggesting a link between tumour burden and enzyme activity. This assumption was further strengthened in those six patients followed lengthwise with regard to serum sialyltransferase activity. Concomitantly with objective evidence of change in tumour burden they showed corresponding alterations in sialyltransferase activity. The determination of sialyltransferase and fucosyltransferase activity in serum may be an additional contribution to refine initial assessment and follow-up of individual patients with multiple myeloma.