The expression of three lysosomal cysteine protease activities, cathepsins B, H, and L, was examined during differentiation of L6 rat myoblasts. Analyses of intracellular levels of these proteases in unfractionated homogenates prepared from cells at different stages of growth and in parallel HPLC-fractionated samples demonstrated a fusion-related increase in all three cathepsins. Analyses of total levels of endogenous inhibitor activity against purified cathepsin B demonstrated a threefold increase in the ratio of protease to inhibitor during myoblast-myotube formation; however, levels of inhibitor activity remained constant. Extracellular levels of cathepsin B, H, and L activities were also examined in the serum-free defined media of differentiating L6 cells. These studies demonstrated a fusion-related increase in extracellular levels of acid/pepsin-activated (i.e., latent) cathepsin L. While increases in intracellular and extracellular levels of cathepsin activities were temporally related to the fusion process, fusion may not be a prerequisite for increased expression, since the nonfusing L6 variant L6-D3 demonstrated high levels of intracellular cathepsins B and L and extracellular latent cathepsin L activities throughout growth. Taken together, these results support the hypotheses that fusion or fusion-related processes play an important role in the controlled expression of cathepsins in L6 myoblasts and that cathepsins, in turn, play an important role in myoblast-myotube differentiation.