Incorporation of [14C]-fucose into the endogenous acceptor of serum and into the exogenous acceptors desialofetuin (alpha 2-fucosyltransferase) and desialodegalactofetuin (alpha 3-fucosyltransferase) was determined in controls and in patients with various malignant diseases. Seventy-five percent of untreated cancer patients showed increased incorporation rates into the endogenous acceptor of serum. Increased rates of alpha 2-fucosyltransferase activity were observed in 67% and of alpha 3-fucosyltransferase activity in 49% of patients respectively. After successful chemotherapy a fall of fucosyltransferase activities could be observed and in non-responding patients an increase of fucosyltransferase activities during and after therapy was found. Determination of fucosyltransferase activities might provide an additional biological marker in monitoring cancer patients.