The healthy action of probiotics is not only due to their nutritional properties and their influence on the gastrointestinal environment, but also to their action on the immune system. The aim of the present study was to determine if 6 weeks of probiotic intake would be able to modulate the immune system in women who had recently delivered and were breast-feeding. The design consisted of a randomised, controlled and double-blind nutritional intervention study with parallel groups with a sample size of 104 women. The main variable is the T helper type 1/T helper type 2 (Th1/Th2) profile determined by measuring interferon-γ (Th1) and IL-4 (Th2) values in peripheral blood by flow cytometry. The modifications of cytokines were evaluated in maternal milk by cytometric bead array in a flow cytometer and ELISA at three stages of breast-feeding: colostrum, early milk (10 d) and mature milk (45 d). Additionally, the anthropometry and infectious and allergic episodes in the newborn were followed up throughout the first 6 months of life. After the consumption of milk fermented with Lactobacillus casei during the puerperium, we observed a non-significant increase in T and B lymphocytes and a significant increase in natural killer cells. A decrease in the pro-inflammatory cytokine TNF-α in maternal milk and fewer gastrointestinal disturbances were also observed in the breast-fed child of the mothers who consumed L. casei. The intake of milk fermented with L. casei during the lactation period modestly contributes to the modulation of the mother's immunological response after delivery and decreases the incidence of gastrointestinal episodes in the breast-fed child.