Eighteen mid-lactation multiparous Holstein cows (n = 9/group) were used to determine the effects of a Saccharomyces cerevisiae fermentation product (NTK, NutriTek, Diamond V, Cedar Rapids, IA) on the response to a mastitis challenge. Cows were fed the control diet (CON) or CON supplemented with 19 g/d NTK for 45 d (phase 1, P1), and then infected in the right rear quarter with 2500 cfu of S. uberis (phase 2, P2). Antibiotic treatment was started 36 h post-challenge until the end of P2 (9 d post challenge). Milk yield (MY) and DMI were recorded daily. Milk samples for somatic cell count were collected 3 times daily, and rectal and udder temperature, heart and respiration rate were recorded every 6 h during the challenge period. Blood samples for metabolites and immune function analysis were collected at 0, 15, and 36 h post-challenge. Data were analyzed by phase using the PROC MIXED procedure in SAS. Cow was used as random effect, while diet, time, and their interaction were used as fixed effects. DMI and MY were not affected by diet (P > 0.05) in P1, but an interaction of diet ´ time was recorded in P2 (MY, P = 0.01; DMI, P = 0.11) indicating a better recovery from the challenge in NTK compared with CON. NTK significantly reduced (P2, P < 0.05) the somatic cell score and temperature of the infected quarter during the challenge, while rectal temperature was significantly reduced (P2, P < 0.05) at the 24 h mark. No effects (P2, P > 0.05) were recorded for circulating neutrophil and monocyte oxidative burst activity; however, NTK reduced (P2, P < 0.05) their activation as indicated by lower phagocytosis response compared with CON at 36 h post challenge. Furthermore, NTK cows had greater (P2, P < 0.05) plasma concentrations of Ca (P2, P < 0.05), suggesting a better systemic inflammatory status, supporting the hypothesis of a better control of the infection at the level of the mammary gland. Overall, results indicate a protective effect of NutriTek supplementation on udder and systemic health during mastitic events.