Heifer calves (n = 70) were used from birth to wk 18 to evaluate the effect of a feed supplement on the immune system. Calves were randomly assigned to (1) control with no supplement (CON; n = 35) or (2) supplemented with Probisan, a lactic acid bacteria and non-bitter fermented yeast product (PRO; n = 35). Calves were in individual hutches until weaning (wk 8), and received milk, ad libitum calf starter, and fresh water. Probisan was added to milk (3 g/d) from d 3 until weaning. From wk 9 to 18, calves were grouped (5 calves) in larger hutches and fed hay and concentrate mixture ad libitum with Probisan added to the concentrate mixture (1.5 g/kg). Body weight was recorded weekly. Calves were vaccinated intranasally at wk 9 with bovine respiratory syncytial virus (BRSV), infectious rhinotracheitis virus, and parainfluenza-3 virus (Inforce 3, Pfizer Animal Health). Nasal secretions were collected from 46 calves (n = 23 per treatment) at wk 16 for the analysis of IgA and BRSV-IgA. Additionally, blood was sampled at wk 17 for the analysis of IgG, and ex vivo E. coli and Salmonella lipopolysaccharide (LPS) challenges. Blood level of IL-1β was measured by ELISA to evaluate the inflammatory response to LPS. Body weight data were analyzed by MIXED procedure with repeated measures, whereas immune data were analyzed by GLM procedure (SAS 9.4). No differences (P > 0.10) were found between treatments for average body weight from wk 0 to 8 (68 ± 0.8 kg) or from wk 9 to 18 (137 ± 1.6 kg). Although blood IgG values were similar (P > 0.10) between treatments (30.1 ± 6.6 mg/mL on average), nasal secretions of PRO contained greater (P < 0.05) IgA (12.5 ± 0.80 vs. 9.3 ± 0.71 mg/mL) and log₂ antibody titer of BRSV-specific IgA (5.63 ± 0.50 vs. 4.60 ± 0.31). Additionally, PRO had lower (P < 0.05) inflammatory response to E. coli (1,413 ± 200 vs. 2,793 ± 405 pg IL-1β/mL) and Salmonella (1533 ± 265 vs. 2797 ± 540 pg IL-1β /mL) LPS. Overall, the greater total and specific IgA in nasal secretions together with the lower inflammatory response to LPS suggest an improvement in the immune status of heifer calves supplemented with Probisan.