The objectives were to investigate the function of the small subunit in the calpain system by expression of the autolytic form of this subunit in L8 myoblasts. Rat post-autolysis small subunit (21 kDa) cDNA expression plasmid was transfected into L8 myoblasts and selected by G418 containing medium. The concentrations of cytosolic -calpain in transfected cells, SS2 and SS3, were found to be 15.7 and 17.3% higher than that in L8Neo control cells, and the concentrations of cytosolic m-calpain in SS2 and SS3 cells were 23.3 and 16.6% higher than that in control cells (L8Neo). The half-life of -calpain in SS3 cells (36.5 h) was longer than that in L8Neo cells (32.4 h), while the half-life of m-calpain in SS3 cells (40.1 h) was longer than that in L8Neo cell (37.5 h). These results indicated that the expression of truncated small subunit increased the stability of - and m-calpain large subunits in cytosol.