The alpha-L-fucosidase activity was studied in blood plasma and cells. In leucocytes and blood plasma of healthy persons the fucosidase activity varied considerably. The fucosidase activity in blood was not altered under pathological conditions other than fucosidosis. The fucosidase activity did not depend on sex and age of the persons studied. No correlation was observed between the values of the fucosidase activity. Two forms of fucosidase (alpha-L-fucosidase I and alpha-L-fucosidase II) were found in human leucocytes and blood plasma. Isoelectric focusing experiments indicates multiple forms alpha-L-fucosidase from human serum. The possibility is discussed of the use of human blood plasma and cells not only for estimation of the total fucosidase activity but also for studies of different forms of the enzyme.