In the present study 210 lactating cows from different farms located in Behera, Alexandria, El-kalupia and Assiute governorates were investigated for mastitis by california mastitis test. The results of bacteriological examination of milk samples showed that 39.10% of cows were positive. The bacteriological examination revealed that the most predominant microorganisms isolated from cows were Coagulase negative Staphylococci +E. coli + Streptococci spp.. Milk scans results of mastitic cows showed increase in total somatic cell count and milk urea, and decrease in milk fat and protein. The leukogram in mastitic cows showed significant increase in total leukocytic count accompanied with neutrophilia with shift left. The biochemical analysis of serum of mastitic cattle cows showed increase in blood urea, creatinine, calcium, prolactin and globulin while a decrease was noticed in serum potassium, chloride, phosphorous, estradiol, total protein and albumin