Toxoplasma gondii is one of the most prevalent parasitic infections in world. Rh, NED and Me49 are of the most prevalent clonal types of the parasite isolated till now. Differences in pathogenicity and virulence of different types have been investigated in different studies. No controlled study was performed to compare the ability of different types to initiate humoral immune response. We investigated IgG antibody responses of kittens infected with each of these three clonal types. For this, experimental infection was performed using ME49 clonal type of T. gondii and humoral immune response (by measurement of IgG) was detected and compared with the other two clonal types of the parasite. No antibodies were detectable at least until 7 days post infection for types Rh and NED while this period of no response was 19 days for ME49. Serum ELISA indices were significantly higher in kittens infected with Rh and NED tpes in comparison with ME49. The results of this study showed that humoral immune response of cats to ME49 starts with delay and are weaker than two other clonal types.