Abstract Aim: This study was planned for determination the toxic effect of thiamethoxam (TMX) in immunized mice with Brucella abortus culture filtrate antigen (CFBAgs) (as a vaccine) and its role of TMX on decrease activity of B. abortus antigen on eliciting of humoral and cellular immunity. Materials and Methods: To achieve these goals 60 female mice were used, 7-8 weeks age, they were divided equally into three groups (20 in each group) and treated as follows: 1st group: Mice were immunized with CFBAgs intraperitoneally in two doses, 2 weeks intervals with (protein concentration 2 mg\ml), 2nd group: Mice immunized as in the 1st group and was administrated orally with 1/10 lethal dose 50% of TMX (83.7 mg/kg B.W.) for 4 weeks daily, 3rd group was administrated orally with 0.3 ml normal saline served as a control group. At day 28 post immunization (PI) delayed type hypersensitivity (skin test) was done, and serum samples were collected at day 30 (PI) for detection of passive hemagglutination test (PHA); interferon gamma (IFN-γ) which was done by enzyme-linked immunosorbent assay test in addition to phagocytes assay. Results: The results of skin test post injection with soluble antigen of B. abortus intradermally showed a high significantly mean values at p≤0.05 of footpad skin thickness in the 1st group of mice which recorded (0.51±0.002 mm) as compared with the 2nd group of mice which showed (0.08±0.002 mm) after 24 h; the mean values of skin thickness were declined in the 1st mice (0.46±0.002) and 2nd mice (0.070±0.001) at 48 h; control group showed a negative results. These results were agreed with results of serum levels of IFN-γ (pg/ml) that showed that a significant increase the vaccinated 1st group (406.36±1.52), than those values in the 2nd group (151.61±0.89) and negative result in 3rd group (46.47±0.60), in addition to results of PHA test which showed a significant increase in antibody titer in the 1st group (139±12.16) with low level of serum antibody in the 2 nd group (7.66±0.33). Phagocytic ratio results in the 1st group showed an increase to reach (18.55±0.44) than a ratio in the 2 nd group (13.24±0.32) and the control group (5.46±0.25). Conclusion: It was concluded that TMX induced suppression of humoral and cellular immune responses in immunized mice with CFBAgs.