A total of 112 quarter milk samples (QMS) were collected from cows and subjected to California Mastitis Test (CMT) for detection of subclinical mastitis where 85.71% of examined quarter's milk samples reacted positively to CMT. Somatic cell count (SCC) was measured and showed that, the mean standard error of SCC was (548.13 x103±24.96) cells/ ml of examined quarter milk samples. Staphylococcus aureus was isolated from 40.62% of positive CMT bovine QMS. Antagonistic activity of probiotic lactobacilli (Lactobacillus acidophilus and Lactobacillus plantarum) against S. aureus isolated from bovine mastitis. Cell Free Supernatant (CFS) of probiotic lactobacilli in well diffusion method had inhibitory
effect. The effect of L. acidophilus on 20 isolated S. aureus strains was 2strains (10%) gave (++++), 2 strains (10%) gave (+++), 6 strains (30%) gave (++) and 10 strains (50%) gave (+) and the effect of L. plantarum was, 2 strain (10%) gave (+++), 8 strains (40%) gave (++) and 10 strains (50%) gave (+). The effect of lactobacilli in co-aggregation method showed that, the high co-aggregation percentage of L. acidophilus and L. plantarumon 20 strains of S. aureus was 94.63% and 94.61% respectively the effect of L. acidophilus on 20 isolated S. aureus strains by modified double layer was 4 strains (20%) gave (+++),7 strains (35%) gave (++) and 9 strains (45%) gave (+) and the effect of L. plantarumwas, 2 strain (10%) gave (+++), 5strains (25%) gave (++) and 13strains (65%) gave(+)