The Z-disc of striated muscle is degraded by the Ca2(+)-activated proteinase, calpain, during autolysis of muscle fibres. The effect of calpain on proteins in preparations of Z-discs isolated from Lethocerus flight muscle has been studied. Calpain releases alpha-actinin from the Z-disc and digests two hydrophobic proteins associated with the Z-disc, zeelin 1 (35 kD) and zeelin 2 (23 kD). The Ca2+ sensitivity of zeelin digestion is shifted to lower Ca2+ concentrations (within the physiological range) in the presence of the phospholipids phosphatidyl inositol or phosphatidyl choline and diacylglycerol. The release of alpha-actinin is not affected by phospholipid. Preparations of isolated Z-discs have five times as much associated phospholipid (w/w) as myofibrils and the composition of the lipid differs from that of myofibrils. In muscle fibres the action of calpain on zeelins may be controlled by the composition of phospholipid in the fibres as well as by Ca2+.