This investigation was performed to demonstrate the effect of continuous feeding of organic acids and antibiotic supplemented diets on some immunological and biochemical parameters in healthy and Salmonella Typhimurium experimentally infected broiler Chickens. One hundred and twenty one-day old Ross 308 chicks were divided into six equal groups. Group 1 negative control. Group 2: infected broilers with 0.5 mL of 1x10⁸ CFU/mL S. Typhimuriumper os as a positive control. Group 3: infected chickens and treated intramuscularly with 0.1 mL / kg body weight kanamycin for 5 successive days. Group 4 administered with propionic acid by a dose of 2 kg/ ton diet then infected with S. Typhimurium. Group 5: treated with formic acid by 1 mL/ L drinking water then infected with S. Typhimurium. Group 6: administered with propionic acid by a dose of 2 kg/ ton diet + 1 mL formic acid / L drinking water then infected with S. Typhimurium. The results revealed that group 2 infected by Salmonella showedloss of appetite, depression, and diarrhea, with mortality rate up to 20%. Group 3 showed mild clinical signs with 15% mortality rate. Groups treated with organic acid showed mild clinical signs with 10% mortality rate. S. Typhimurium infected chickens (G2) revealed leukocytosis, heterophilia, monocytosis, significant increase in uric acid, creatinine, liver enzymes activities, IgA and IgG and significant decrease in albumin and total antioxidant capacity  (TAOC). While treatment of Salmonella infection with kanamycin, organic acids alone or combination of propionic acid and formic acid resulted in a positive effect in treatment, a significant improvement in some immunological and biochemical parameters and ameliorating the severity of infection.In conclusion, the combination of organic acids revealed the best results as it returned leukogram, biochemical and immunological parameters nearly to their normal levels and decreased bacterial colonization in poultry.