Chitosan/bovine serum albumin (BSA) ultrathin films were fabricated on modified quartz wafer through layer-by-layer deposition．The effects of experimental conditions

such as pH value

molecular weight of CS

ionic strength and polyanions coexisting in BSA solution

on the fabrication and the stability of films were investigated with a radio-labeled method．The results show that the absorbed BSA is almost proportional to the number of assembly cycle except the first 2 cycles

and chitosan／BSA films are favorable to be fabricated in chitosan aq AcOH at pH=5.2 as compared with at pH=1.0．When two different molecular weight (MW) chitosans were used (Mη≈1.2×105

Mη≈1.0×106)

the lower MW chitosan adsorbs more BSA

but less stable ultrathin films are fabricated．Similar results were obtained that the amount of adsorbed BSA is higher hut the films formed are less stable in water than in phosphate-buffered saline (PBS)．The adsorption of BSA is restricted greatly in the presence of poly(sodium styrene-sulfonate) (PSS) mainly because of the competitive adsorption．