In cell culture experiments, the antibodies were used in immunocytochemical assays, where monolayers of HeLa cells infected with T. gondii were incubated with IgY anti-STAg. In these experiments, we observed isolated or grouped intracellular tachyzoites closely to cell nucleus, detected by an intense fluorescence on the whole tachyzoite surface (Fig. 4) . Overlay of fluorescence emitted by the stained parasites and cell nucleus with phase contrast images of the fibroblast monolayer provided a valuable measure of the parasitism of the infected monolayers.