This study was conducted on 50 dairy cows, which were randomly collected from different smallholder farms in Assiut city. 180 milk samples representing 50 dairy cows under test were firstly screened for the presence of subclinical mastitis by using California Mastitis Test (CMT); 58 milk samples yield positive results, and all of these were positive for the bacteriological examination. The isolated bacterial strains (102 isolates) including contagious bacteria representing 75 strains (73.5%) and environmental bacteria 27 (26.5%). 48.3% of milk samples showing mixed infection. The isolated contagious strains were Staph aureus (30 strain), coagulase negative Staph (CNS) 40 strain, Strept agalactia 5, while the environmental bacteria were E coli 10, Enterococcus sp < /em> 8, Klebsiella spp < /em> 2 and Bacillus spp < /em> 7. The electropheritic patteren of milk proteins showed that there were Significant statistical increase in total whey protein, immunoglobulin and albumin in cows' milk samples with sub clinical mastitis. While α–lactalbumin, and β-lactoglobulin showed significant decreased. There were a highly significant decrease in α -casein and қ casein in sub clinical mastitic samples in comparing with the normal milk. In addition, there were a significant decrease in β-casein and non-significant decrease in γ casein milk protein fractions in sub clinical mastitic milk. Recommendations were suggested to eradicate and control subclinical mastitis to decrease the economic losses in lactating dairy cows.