The objective of the reported study was to determine the effect of meat sonication within up to 2 h after slaughter with low-frequency ultrasounds (25 kHz) and at medium vibration intensity (2 W/cm²) on selected properties of myofibrilar proteins of the I-Z-I band isolated from ageing beef. Proteins were isolated directly after sonication as well as after 24, 48 and 72 h of storage at a temperature of +4°C. The isolated protein fraction was determined for: myofibrillar fragmentation index (MFI), content of reactive sulphydryl groups, redox potential, surface hydrophobicity, and proteins capability for fat emulsification. Based on the results of analyses of the isolated proteins of the I-Z-I band, no significant effect of post-slaughter sonication immediately after the slaughter was noted on the content of reactive sulphydryl groups, area of hydrophobic surface, myofibrillar fragmentation index nor redox potential. The results obtained point to a significant decrease in the capability of I-Z-I band proteins isolated from sonicated meat for fat emulsification. The research needs to be continued and extended at a molecular level in order to determine which of the myofibrillar proteins are the most susceptible to the action of ultrasounds, and to identify the mechanism of their action on biologically-complex protein systems in meat.