The immunization of horses with tetanus toxins for the production of antitetanic sera was associated with significant vital changes, a significant rise of body temperature and a slightly significant increase of pulse and respiratory rates in young, middle aged and geriatric horses. All horse groups showed different degrees of local reactions of the sites of injection ‘Of tetanus" toxins; beginning from diffuse, large, hot, tender, and painful swell ang swellings extending from the thoracic inlet to the head at the injected side of the neck which subsided gradually to multiple local abscesses formation at the sites of injections followed by complete sloughing of the skin with ulcer formation covered by creamy yellow pus. The immunological investigations showed significant increase in the total leukocytic count with concomitant increase of both polymorphonuclear and band cells in all horses groups and the quantitative estimation of tetanus toxins antibodies revealed better response of young horses than middle aged and geriatric horses. The hematological investigations revealed a significant decrease in hemoglobin as well as RBCs count with concomitant decrease in MCHC in geriatric horses more than young and middle-aged ones. Significant increase in prothrombin time and significant decrease in activated partial thromboplastin time as well as platelets count were recorded in geriatric horses. Serum biochemistry revealed a significant decrease in total proteins with concomitant decrease in albumin in middle aged and geriatric horses groups. Scrum AST was significantly increased in all horse groups and total bilirubin was significantly increased in middle aged and geriatric horses.