The formation of protein-polymer complexes was studied in an aqueous system using dynamic light scattering (DLS) and static light scattering (SLS) as the main experimental tools. Human serum albumin (HSA) was used as a protein and complexed with four representative water-soluble polymers: poly(N-isopropylacrylamide) (PNIPA), poly(ethylene glycol) (PEG), poly(vinyl pyrrolidone) (PVP), and poly(vinyl alcohol) (PVA). The first three molecular weights were within 420,000-540,000 and the last one was 270,000. The complexation was performed at 25 degrees C in 0.01 M NaCl solution adjusted to pH 3 with HCl as a function of mixing ratio (rm; molar ratio of polymer to HSA). From SLS experiments, we determined the molecular weight of the resulting complexes, from the value of which the number (nb) of bound proteins per polymer was estimated. It was found that each polymer forms an intrapolymer complex over a wide range of rm (1.2 > or = rm > or = 0.01). Then, a marked decrease in nb with increasing rm was found. Over the whole rm range, the HSA-PNIPA complex exhibited a large nb value, as compared with the other three complexes whose nb values at the same rm were close to one another. Both the hydrodynamic radius (Rh) by DLS and the radius of gyration (Rg) by SLS for the complexes of PNIPA, PVP, and PVA decreased and then reached a constant value as nb decreased with increasing rm. In the PEG system, however, there were a few changes in Rh and Rg with nb. The Rg/Rh ratio, as an indication of chain expansion, was found to increase with decreasing nb in the PNIPA system. The complexes of PVA and PVP displayed a similar tendency, although the magnitude of the increasing trend was smaller than that of the PNIPA complex. In contrast, the Rg/Rh ratio of the PEG complex hardly varied depending on nb. These results were discussed in connection with the differences of physicochemical properties among four water-soluble polymers.