Cu2+-mediated complex formation between temperature-responsive poly(N-isopropylacylamide-co-acrylic acid) (CP) and bovine serum albumin (BSA) was investigated in neutral and in physiological salt solutions. Water soluble or insoluble ternary complexes are formed upon addition of divalent copper ions to the solution. Stability of the ternary polycomplexes depends on the composition of the copolymers, concentration of copper ions and ratio of components. Stimuli-sensitive (temperature, pH-and ionic strength-) behavior of CP, CP-Cu2+ and CP-Cu2+-BSA solution was studied. The ternary CP-Cu2+-BSA complexes exhibit reversible temperature-responses in physiological salt solutions at suitable ratios of the components. Thermal collapse of ternary polycomplexes was observed in higher pH solutions than for CP and CP-Cu2+. The negative temperature solubility coefficient in salt containing solutions may be due to cooperative hydrophobic interactions.