The binding of cytochrome c to pH and thermoresponsive colloidal hydrogels was investigated using multiangle light scattering, measuring loading through changes in particle molar mass and root-mean-square radius. Loosely cross-linked microgels [composed of a random copolymer of N-isopropylacrylamide (NIPAm) and acrylic acid (AAc)] demonstrated a high loading capacity for protein. Encapsulation was dependent on both the charge characteristics of the network and the salinity of the medium. Under favorable binding conditions (neutral pH, low ionic strength), microgels containing the highest studied charge density (30 mol % AAc) were capable of encapsulating greater than 9.7 x 10(5) cytochrome c molecules per particle. Binding resulted in the formation of a polymer-protein complex and condensation of the polymer. Anionic microgels demonstrated a change in density similar to 20-fold in the presence of oppositely charged proteins. These studies of cytochrome c encapsulation represent a significant step toward direct measurement of encapsulation efficiency in complex media as we pursue responsive nanogels and microgels for the delivery of macromolecular therapeutic agents.