This research was carried on 92 samples from clinically mastitic cows collected from 4 different places (farms) at Sharkia governorate and tested for detection and determination of the incidence of pathogenic bacteria among cows. The prevalence of the major contagious pathogenic bacteria of mastitis were E. coli, Staphylococcus aureus, Streptococcus agalactia and Klebsiella pneumonae in a ratio of 51.1%, 21.7%,18.5%, 7.6%, 1.1% respectively. Mixed infection was observed in 21.37%, while bacteriologicaly negative samples were absent. Sensitivity test against isolated strains in vitro appeared that they were 207 Assiut Vet. Med. J. Vol. 52 No. 109 April 2006 susceptible to cephalosporin, gentamycin, enrofloxacin, and amoxicillin. This retrospective study was prompted by findings of haematological and biochemical examinations, and was adopted for detection and diagnosis of mastitic cows.