Noninfected snails of the Biomphalari and Bulinus types (intermediate hosts for Schistosoma mansoni and Schistosoma hematobium , respectively) can oxidize instantly and intensively p -phenylenediamine dihydrochloride and its derivatives to the corresponding colored trivalent nitrogen-free radicals. This reaction fails to occur with snails infected with miracedia of Schistosomes . The reaction is inhibited through inhibitors specific for enzymes of the protoporphyrin type, also through the potent molluscicide (bayluscide). Studies revealed the enzyme involved in the reaction is the peroxidase, along with the catalase. Both enzymes were found to be present in both types of snails.