Mouse macrophages and LA-9 cells were infected with Toxoplasma gondii. The percentages of cells containing parasites inside cytoplasmic vacuoles and with parasites attached to the cell surface were determined. Treatment of macrophages or LA-9 cells with either cytochalasin B or 4 degrees C before interaction with T. gondii decreased, but did not totally prevent their infection by the parasite. Observation of the host cell/parasite interaction by scanning electron microscopy revealed images of parasite penetration into cells, even when the cells had been incubated at 4 degrees C or in the presence of cytochalasin B. Two types of cytoplasmic vacuoles, which contained parasites were seen in thin sections of infected macrophages: in one type there was a small space between the membrane of the parasite and the membrane of the vacuole; in the second type the vacuole was very large and contained tubular structures. The results obtained are interpreted as indicative that two mechanisms are used by T. gondii to enter vertebrate cells.