The soluble intracellular protease was partially purified from L. donovani promastigotes. The activity of this enzyme increased with increase in temperature from 25 degrees C to 37 degrees C and was active optimally at 70 degrees C. This protease activity appeared to be decreased due to heat-shock of the promastigotes for 4 h at 37 degrees C and increased due to nutrient starvation. Inhibition of the protease by p-chloromercuribenzoate and iodoacetamide suggested that this enzyme could be a thiol protease.