The ethanolic phosphotungstic acid (E-PTA) technique was used to locate basic proteins in trophozoites ofToxoplasma gondii, obtained from the peritoneal exudate of infected mice. The reaction was observed mainly on the conoid, rhoptries, and micronemes of both extracellular and intracellular parasites. The strongest reaction was observed on rhoptries. Nuclei and ribosomes from parasites and host cells also reacted. The role of basic proteins on rhoptries and micronemes is discussed with a view on host-cell penetration by the parasite.