The objective of this study was to evaluate the effects of a direct-fed microbials (DFM; Bacillus pumilus 8G-134) fed during the transition period on health of Holstein cows. Forty-three multiparous cows were fed a total mixed ration and assigned to 2 treatments in a randomized completely block design. Cows in the direct-fed microbial treatment (DFMt, n = 21) received 5.0 × 10⁹ cfu of B. pumilus direct-fed microbial in 28g of maltodextrin carrier, whereas cows in the control treatment (CON, n = 22) received 28g of maltodextrin carrier alone. Treatments were top-dressed daily. Treatments were applied from 21 d before calving to 154 d after calving. Blood samples were collected on d 5 and 14 after calving. Cows that had nonesterified fatty acids (NEFA) serum concentrations higher than 0.7 mEq/L were classified as high (HNEFA). Cows that had blood β-hydroxybutyrate (BHBA) concentrations higher than 1.2 mmol/L were classified as experiencing sub-clinical ketosis (SCK). Cows that had serum haptoglobin concentrations higher than 20 μg/mL were classified as positive (POS). The immunoglobulins, IgA, IgG, and IgM were quantified in milk collected during the first week after calving and serum. Statistical analysis was performed using the MIXED, GLIMMIX and FREQ procedures of SAS. Cows on CON tended to have greater odds (OR = 3.55; P = 0.09) of being classified as POS than CON cows on d 14. Treatment DFMt had higher (584 ± 34 μg/mL; P = 0.03) IgA concentrations in milk than CON (478 ± 35 μg/mL) cows on the first week after calving. Cows on CON tended to have greater odds of being classified as HNEFA than DFMt cows on d 14 (OR = 3.21; P = 0.07). Cows on CON tended to have greater odds of being classified SCK than DFMt cows on d 5 (OR = 3.85; P = 0.06). Cows on CON tended to have higher odds for lower (<2) FS than DFMt cows (OR = 1.03; P = 0.08). In conclusion, cows receiving DFMt tended to have lower incidence of SCK and to have higher immunity than cows receiving CON. Therefore, supplementation with DFMt seems to contribute for a sound transition period for dairy cows.