Schistosoma mansoni depends for its survival on glycolysis. Two glycolytic enzymes, glyceraldehyde-3P-dehydrogenase and triose-phosphate dehydrogenase, found in both the adult and schistosomular tegument, have been reported to confer partial protection against cercarial infection. This paper describes the immunogenic properties of phosphofructokinase (PFK), a rate-limiting enzyme of glycolysis, and its localization in the tegument and adjacent tissues. Recombinant schistosome PFK was used as antigen. A polyclonal antibody against purified PFK from Fasciola hepatica was affinity purified using recombinant PFK and used in combination with immunogold labelling to identify PFK by transmission electron microscopy in cryosections. In both adult worms and in schistosomula most immunogold label localized in the cytoplasmic syncytial region with less being found in the tegument. There was no significant PFK localization within or external to the outer membrane. Sera from mice immunized with recombinant S. mansoni PFK with Freund's adjuvant or alum plus rIL-12 demonstrated high titres of anti-PFK IgG, but no protection against cercarial infection. Sera from mice that were acutely or chronically infected or multiply exposed to irradiated cercariae did not recognize recombinant schistosome PFK in either Western blotting or ELISA. Similarly, sera from humans infected with S. mansoni did not recognize PFK. We conclude that in spite of the high immunogenicity of rPFK in mice, it is not a significant immunogen during the course of infection and does not confer protection from schistosomiasis. One main difference between PFK and the other 2 glycolytic enzymes seems to be the inaccessibility of PFK to the outside surface of the tegument.