Changes in the Golgi bodies and in hydrolytic enzymes were observed in parasitic ciliates (Nyctotheroides puytoraci) and flagellates (Opalina sudafricana and Protoopalina sp.) after the administration of a single dose of 0.5 mg beta-naphthylamine (BNA) to their host, Bufo regularis. The experiment was carried out during the host's pre-breeding season, from November to February (when only trophozoites are available); the trophozoites were examined 21 days after the injection of BNA. Use of the silver impregnation technique showed an increase in the size of the granular argentophilic Golgi bodies in all three trophozoites after administering BNA to their host. In addition, a larger number of acid phosphatase and nonspecific esterase granules was found in the endoplasm of the three parasites after the injection. The localization of acid phosphatase and nonspecific esterase was found to be similar to the distribution of the Golgi bodies in both the control and the treated preparations. The results could be useful, in that some protozoans could be employed for the quick detection of chemical carcinogenicity if changes in their hydrolytic enzyme content are used as a diagnostic feature.