Abstract: Mouse NB2a/dl neuroblastoma cells elaborate axonal neurites in response to various chemical treatments including dibutyryl cyclic AMP and serum deprivation. Hi-rudin, a specific inhibitor of thrombin, initiated neurite outgrowth in NB2a/dl cells cultured in the presence of serum; however, these neurites typically retracted within 24 h. The cysteine protease inhibitors leupeptin and N-acetyl-leucyl-leucyl-norleucinal (CI; preferential inhibitor of micromolar calpain but also inhibits millimolar calpain) at 10-6M considerably enhanced neurite outgrowth induced by serum deprivation, but could not induce neuritogenesis in the presence of serum. A third cysteine protease inhibitor, N-acetyl-leucyl-leucyl-methional (CII; preferential inhibitor of millimolar calpain but also inhibits micromolar calpain), had no detectable effects by itself. Cells treated simultaneously with hirudin and either leupeptin, CI, or CII elaborated stable neurites in the presence of serum. Cell-free enzyme assays demonstrated that hirudin inhibited thrombin but not calpain, CI and CII inhibited calpain but not thrombin, and leupeptin inhibited both proteases. These results imply that distinct proteolytic events, possibly involving more than one protease, regulate the initiation and subsequent elongation and stabilization of axonal neurites. Since the addition of exogenous thrombin or calpain to serum-free medium did not modify neurite outgrowth, the proteolytic events affected by these inhibitors may be intracellular or involve proteases distinct from thrombin or calpain.