Recently, the Ultrafiltrator has been described, in the literature, which allows the ultrafiltration of small molecules in minute blood samples under anaerobic conditions with filters of any pore size. This apparatus has been tested for the separation of free tryptophan in heparinised and EDTA-plasma and in whole blood. At the same time, free tryptophan has been analysed after ultrafiltration with CF-50 A and CF-25 membrane cones (Centriflo). The results obtained by these two techniques are compared. Finally, a comprehensive review of other methods reported in the literature, concerning the isolation of free tryptophan, is presented.