The aim of this study was to determine the effect of aflatoxin B1 on redox status and incidence of mastitis in dairy cows. The current study was done on 32 lactating Holstein cows; the first group (10 cows) were received ration, with aflatoxins not exceed permissible limits, while the second group (22 cows) received ration containing high levels of aflatoxins. Two blood samples were collected from each cow the first sample was taken heparinized for plasma separation and estimation of oxidant/antioxidant parameters (total antioxidant capacity, glutathion peroxidase, catalase & superoxide dismutase, malondialdehyde, nitric oxide and determination of the cell mediated immune response. The second blood sample was taken for serum separation for evaluation of copper & zinc levels. In the same time, after California Mastitis Test (CMT) was done, double milk samples were taken aseptically from 32 lactating Holstein cows. One sample was examined for bacteriological studies while the other was examined for milk fat %, protein % and somatic cell count (SCC). The results of second group showed significant reduction at (p < 0.05) in antioxidant parameter; total antioxidant capacity, glutathion peroxidase, catalase & superoxide dismutase, while increased significantly at (p < 0.05) in oxidant stress parameters; malondialdehyde, nitric-oxide. In contrast copper and zinc showed non significant decreased. Concerning Cell mediated immune response; phagocytosis (%), killing (%) of polymorph-nuclear cells and lymphocyte stimulation index the data revealed a significant reduction at(p < 0.05). As a result of the impaired celluar immunity, all cows in second group suffered from subclinical mastitis The changes in CMT scores and SCC levels in cow's milk in subclinical infections were due to some important mastitis pathogens., S.aureus was the most prevalent organism (31.25%) followed by E.coli (25.0%) then str. dysgalactia (15.63%) and Coagulase Negative Staphylococci (CNS) was (15.63%). Significant decrease at (p < 0.01) was observed in milk fat % and protein %. We observed that aflatoxin B1 increase oxidative stress, immunosupprion, which may decrease resistance to infection disease, predispose cows to incidence of mastitis resulting in increase SCC, decrease milk fat % and protein %.