We investigated the factor which increased the maximum value of the Mg-ATPase activity of myofibrils existing at low KCI concentrations during meat conditioning. On the treatment of myofibrils with the solution extracted with the buffer of pH 7.2 from muscle, the Mg-ATPase activity in the presence of 0-0.15 M KCI increased time-dependently. This change was most remarkable in the range of pH 5.6-7.0. Trypsin treatment of the extract abolished such effect, suggesting that the responsible factors were proteins. The fractionation of the extract with isoelectric focusing demonstrated that the factors were basic proteins (pI 8.3-9.6). The treatment of myofibrils with those basic proteins under various conditions suggested that the time-dependent adhesion of those basic proteins, through a denaturation at around pH 5.5, to myofibrils was assumed to raise the Mg-ATPase activity. Analysis of myofibrils prepared from rabbit muscles stored at 0 degreesC for 12 days postmortem showed the appearance of the 35,000 Da protein, accompanied the increase in the Mg-ATPase activity. Therefore, the adhesion of this protein to myofibrils in situ probably caused the increase in the Mg-ATPase activity. Successive treatment with the basic protein and the crude cathepsin increased the dependency of the Mg-ATPase activity on KCI concentrations and the maximum value of the Mg-ATPase activity. Therefore, the coordinate action of a basic 35,000 Da protein and cathepsins was presumed to induce the changes in the Mg-ATPase activity of myofibrils during meat conditioning. The basic protein was concluded to be glyceraldehyde-3-phosphate dehydrogenase (its subunit molecular mass: 35,000 Dal, since the incubation of myofibrils with its commercial preparation raised the Mg-ATPase activity of myofibrils. (C) 2000 Elsevier Science Ltd. All rights reserved.