An acidic proteolytic enzyme which digests host haemoglobin can be isolated and purified from schistosomes. This small glycoprotein is an allergen which sensitizes the host, as shown by immediate hypersensitivity reactions. These are specific for either Schistosoma haematobium or S. mansoni and can be demonstrated by mast cell degranulation in mice or by intradermal skin tests in monkeys. Although high levels of total IgE may be found in acute and chronic schistosomiasis, there was no evident relationship between the worm burden in monkeys and immediate hypersensitivity reactions to either purified enzyme or crude schistosomal extracts. It is suggested that an in vivo correlation between worm burden and manifestations of the allergic response may be perturbed by high titres of non-specific IgE or other homocytotropic antibodies, thus accounting for false negative skin test reactions. Alternatively, a return to low or subnormal IgE levels may allow the restoration of the allergic response, giving rise to false positive reactions. Purified schistosomal antigens offer certain advantages over crude skin test preparations in terms of uniformity of antigen content, dosage and specificity. In addition, the enzyme may represent a species-specific tool for new immunochemical analyses of schistosomiasis.