The initial association of tachyzoites of Toxoplasma gondii with a host cell induces an endocytic process which leads to the formation of a vacuole known as the parasitophorous vacuole (PV). We analyzed the parasite-host cell interaction process using either parasites or host cells whose membrane was previously labeled with probes specific for proteins, sialoglycoconjugates and lipids, and then allowed to interact for periods varying from 5 minutes to 24 hours. The fate of the fluorescents probes was followed by confocal laser scanning microscopy. In host cells previously labeled with PKH26, FITC-Thiosemicarbazide or DTAF, which label membrane proteins, siloglycoconjugates and lipids, respectively, a uniform labeling of the cell surface was observed before interaction. When allowed to interact with T. gondii, labeling for PKH26 and DTAF, but not for FITC-Thiosemicarbazide, was observed initially at the region of contact between the two cells and subsequently on the membrane lining the PV and the intravacuolar parasites. These observations show that some, but not all, membrane components contribute to the formation of the PV membrane. Previously labeled parasites attach to the host cell surface but lose the fluorescent probes during the invasion process so that no labeled parasites were seen within the PV. These observations point to the existence of a dynamic process of membrane-associated components of the parasite and host cell during the interaction process.