Promastigotes of all pathogenic Leishmania species secrete acid phosphatase (SAcP) activity during their growth in vitro. It has been suggested that this enzyme may play a role in the survival of the parasite within its sandfly-vector host. To carry out such functions, SAcP would have to be relatively resistant to endogenous sandfly gut-proteases. Therefore, the current study was undertaken to ascertain whether L. donovani SAcP activity was affected by treatment with various proteases. Native L. donovani SAcP was treated with a variety of serine-, thiol-, metallo- and mixed-proteases and subsequently assayed for enzymatic activity. Of the eleven proteases tested, only bromelain and subtilisin treatments caused a pronounced reduction in SAcP activity. Treatment of SAcP with seven out of the remaining nine proteases, resulted in an overall enhancement in SAcP enzymatic activity ranging from ∼ 10% (e.g. with trypsin) to ≥ 90% (e.g. with ficin). The resistance of the Leishmania SAcP to various proteases may prolong its functional life within the sandfly gut and help to facilitate parasite infection in this host.