A series of biodegradable heparin/ampholytic chitosan (HP/ACS) complexes were developed for pH- sensitive release of protein drugs．ACS was synthesized by using acrylic acid (AA) and chitosan

and the substitution degree (SD) of AA can be modulated by the AA/chitosan feed ratio．The complexation between heparin and ACS was studied by colloid and turbidity titration．It was found that HP and ACS can form complexes within a certain pH range and the critical pH value for complexation (pHФ) is related to the SD of AA．The larger the SD

the higher the pHФ value．Bovine serum albumin (BSA) was selected as a model protein

and its entrapment and release behavior from HP/ACS complexes were investigated．The results show that the entrapment efficiency of BSA is very high(～100％)

and the BSA release is extremely pH-dependent．The transition of BSA release can occur within a rather narrow pH range(Ф) can also be modulated by SD．It seems that the BSA release transition is correlated with HP/ACS complexation．In addition．the sustained release of BSA from the complex can be achieved at pH 7.4 and the releabe duration can be up to 15 days．