ABSTRACT: The effects of ionic strength (0, 0.3, and 0.6 M KCl) and temperature (4 and 25 °C) on the interaction between fish myoglobin and myofibrillar proteins were investigated in a model system. Increases in the relative content of bound myoglobin and metmyoglobin formation in myoglobin-natural actomyosin (NAM) mixtures with concurrent decreases in whiteness and Ca2+-ATPase activity were observed with increasing ionic strength (P < 0.05). The relative content of bound myoglobin and the oxidation of oxymyoglobin were generally greater at 25 °C than at 4 °C (P < 0.05). Binding of myoglobin to NAM resulted in decreased whiteness (P < 0.05). Ca2+-ATPase was not affected by temperature (P > 0.05). SDS-PAGE patterns of protein samples suggested that myoglobin-NAM interactions did not involve disulfide bonds. The formation of high-molecular-weight aggregates (>206 kDa) was observed and was more pronounced at higher ionic strength and higher temperature.