Abstract: Tetra-N-acetyl-chitotetraose, penta-N-acetylchitopentaose and hexa-N-acetyl-chitohexaose, ones of the water-soluble chitin-oligomers, may be obtained in improved yields, when finely ground chitin is quickly and intimately mixed with a concentrated hydrohalogenic acid containing a particular proportion of the hydrogen halide per a unit quantity of the chitin under the irradiation with ultrasonic waves and also under the agitation by mechanical stirrer, followed by hydrolyzing the chitin in the resulting homogeneous mixture comprising the chitin and the concentrated hydrohalogenic acid while said homogeneous mixture is further continuously irradiated with the ultrasonic waves and also agitated by the mechanical stirrer. From the aqueous phase of the hydrolyzed reaction mixture are recovered the desired water-soluble chitin-oligomers which are each useful as an immunopotentiating, antitumor agent or antifungal, antibacterial agent.