The tegument of Schistosoma mansoni in situ in mesenteric veins was observed in mice with and without horseradish peroxidase injections. No evidence of incorporation of horseradish peroxidase into the tegument was noted after 45 minutes of high serum levels of peroxidase. Scant endogenous peroxidase was noted in multilaminate vesicles of the tegument. No evidence of significant endothelial changes, activation of coagulation or cellular response was associated with worms in situ. Endothelia in adjacent worms showed no increased permeability to horseradish peroxidase.