Chitosan was selectively N-acylated with various carboxylic anhydrides, e.g., acetic, propionic, n-butyric, n-valeric and n-hexanoic anhydrides, in the presence of methanol. The degree of N-acylation of about 20-50% was obtainable without occurrence of gelation by using carboxylic anhydrides of 0.3-1.2 mol per glucosamine residue. In vitro blood compatibility tests of N-acyl chitosans were performed by rheological measurement, blood clotting test and scanning electron microscopic observation for human blood and plasma protein. The rheological measurement of coagulation of plasma protein, considering the shear flow effect of blood, gave precise and quantitative results compared with other methods. N-Acyl chitosans showed more blood compatible properties than N-acetyl chitosan and, in particular, N-hexanoyl chitosan was the most compatible. Enzymatic degradation was also investigated by adding a lysozyme solution to the N-acyl chitosan solution and film, incubating at 37 degrees C. N-Acyl chitosans had as high a susceptibility to lysozyme as N-acetyl chitosans. It was considered that the amount of derivatized groups and the physical form of N-acyl chitosans contributed to biodegradability. The molecular weight (Mw) of the material liberated from the N-acyl chitosan film by the action of lysozyme was 2 x 10(4) - 10 x 10(4).