The diffusion of sodium chloride and acetic acid into herring muscle and their subsequent effects on the myotibrillar proteins were studied at different temperatures and fish:cure ratios. Acid was found to penetrate tissue more quickly than salt. The acid and salt cause an initial "hardening" of the tissue, the extent of which is proportional to the concentration of each, but it is considered that the resultant fall in pH activates muscle cathepsins. SDS PAGE indicated that proteolysis of the myosin heavy chains was concomitant with subsequent tissue softening. Electron microscopy detected little disintegration of myofilaments, but extensive break-up of Z-lines.