The present study was designed to infect lambs with H. contortus. The eggs appeared in feces of experimentally infected lambs 21 — days post-infection and the highest level of eggs/gram feces “EPG" was recoreded on day 38 post-infection (p.i.). Experimentally infected sheep with H. conortus showed significant decrease in Hb%, R. B. Cs count and PCV accompanied with leucocytosis. Differential leucocyte counts revealed neutrophilia and eosinophilia with decrease in lymphocytic counts, while basophils and Monocytes were not affected. After treatment (45 days pi. with levamizole), the blood picture had improved and reached the normal level 35 days after treatment. biochemical analysis of serum samples revealed decrease in total protein, albumin and A/G ratio, while serum globulin level was increased. There was also decrease in serum calcium and Phosphorus levels. The biochemical changes improved and returned back to their normal levels 35 days post treatment (p. i.). Sera from experimentally and naturally sheep haemonchsis were used to evaluate the diagnostic sensitivity of egg and crude H. contortus antigens using ELISA as compared to IHA. The present study revelaed that, the crude antigen gave the highest sensitivity value with ELISA 100% and IHA 91.67%. This result indicated that the crude antigen was the most suitable one for diagnosis of sheep haemonchosis especially with ELISA. Histopathological changes of abomasum from experimentally infected lamb cleared dilatation and engorgement of serosal blood vessels together with submucosal leucocytic aggregation as well as the mucosal epithelium was distructed and sloughed.