Protein‐protein interaction of bovine natural actomyosin (NAM) was studied by means of optical density changes resulting from discrete particle formation in the temperature range of4°C to 70°C. From Arrhenius plots, the apparent heat of activation (ΔHa) at pH 5.5 (17.1 kcallmole) was significantly (P<0.05) lower than activation energies in the pH range of 6.0 to 7.5. The lower Δ Haresulted in initiation of protein‐protein interaction at a temperature near 16°C at pH 5.5, whereas interaction did not proceed until the temperature approached 37°C at pH 6.0 and above. Derivative curves (dOD/dT) at pH 5.5 and 6.0 showed two distinct NAM thermal transition regions. Tm1occurred at 43.0°C at pH 5.5 and 48.5°C at pH 6.0, with the 5.5°C difference possibly arising from effects of proton binding in altering protein conformation. Only a 1.5°C difference in Tm2(56.0°C at pH 5.5 versus 57.5°C at pH 6.0) was found. Although the overall heat‐mediated NAM aggregation (in dilute solution) was found to follow first order kinetics by two evaluation methods, the existence of two thermal transitions supports a two‐step reaction mechanism proposed for the formation of protein gels (in higher conce