The proteolytic activities present in adult Necator americanus excretory-secretory products have been assessed using biologically relevant, naturally occurring substrates (haemoglobin and fibrinogen) and a number of synthetic fluorogenic and chromogenic substrates. One broad peak of activity was observed against haemoglobin in the pH range 5 to 7, with maximum activity at pH 6.6, while fibrinogenolytic activity was shown to be greater at pH 3.5. Inhibition studies against haemoglobin, fibrinogen and synthetic substrates using a battery of appropriate protease inhibitors indicated the presence of a mixture of aspartyl, cysteinyl and serine proteases. Metal ion (Ca2+, Zn2+ and Fe2+) stimulation was demonstrated, with stimulation by Zn2+ being the most marked. These results are discussed in the context of recent developments in the field of parasite proteolytic enzymes, where they have been suggested as targets for immuno- and chemotherapy.