Toxoplasma gondii is an obligate intracellular protozoan parasite. The objective of the present study was to examine the behaviour of Toxoplasma gondii RI-I Ankara strain tachyzoites in a cell Culture environment. The study represents the first step in determining whether T. gondii RH Ankara strain tachyzoites, grown in cell culture, are of sufficient quality to allow cessation of in vivo tachyzoite production for diagnostic assays. In the present Study, T. gondii RI-I Ankara strain tachyzoites were continuously produced ill myeloma X63.Ag8.653, HeLa, Hep-2, and Vero Cell Culture, for 2 months. The average size of the tachyzoites was 3 x 5-7 mu m prior to the first inoculation but after continuous production, a marked decrease was noted in average tachyzoite size. The smallest tachyzoite size, was 1 x 2-1 mu m after 2 months, in myeloma cell cultures even though the yield of tachyzoites increased. With other cell cultures, tachyzoite yields were not as high as myeloma cell Culture tachyzoites size, averaging 2 x 3-8 mu m after 2 months, was although decrease in size was less. The smallest decrease in tachyzoite observed in tachyzoites produced in HeLa cell Cultures. A virulence assay in small groups of BALB/c mice, using tachyzoites derived from Cell Cultures, was also conducted. The preliminary results of the virulence assay suggest that as the size of the tachyzoites decreased, the virulence in mice decreased. Future research will focus on the effect of the size of cell culture-derived T. gondii RI-I Ankara strain tachvzoites on the virulence, protein expression, and the reliability of diagnostic assays. Ultimately, the behaviour of tachyzoites from various T. gondii strains will be observed in cell culture to determine if size is altered.