Ultrastructural and biochemical changes of bovine psoas muscle produced by leukocyte lysosomal hydrolases in vitro were studied at two different temperature, 37 and 4℃ by SDS-polyacrylamide gel electrophoresis, scanning and transmission electron microscopy. Severe degradation of endomysial and sacolemmal connective tissue, transverse ridges, Z-lines and M-lines were observed in lysosomal enzyme treated muscle by electron microscopy. Myosin heavy chain and α-actinin were observed to be susceptible but actin and troponin-T was very stable to lysosomal hydrolases by SDS-polyacrylamide gel electrophoresis.