This study aimed to evaluate the effects of ingesting yogurt fermented with Lactobacillus delbrueckii subsp. bulgaricus (OLL1073R-1) on the immune function of healthy university men track and field athletes. Study design Randomized, double-blind, placebo-controlled, parallel-group study. A total of 37 track and field athletes aged ≥18 years were randomly assigned into two groups. For 2 weeks, two bottles of yogurt fermented with OLL1073R-1 and Streptococcus thermophilus OLS3059 or placebo sour milk were ingested daily to the participants. During the intake period, a 1-week training camp was held and participants were subjected to strenuous exercise. Natural killer (NK) cell activity, which is the primary endpoint, was significantly lower in the placebo group after ingestion than that at baseline; however, it remained unchanged during the pre-exercise level of the yogurt group. The two-way repeated measures analysis of variance showed an interaction effect in the NK cell activity change (P=0.018) and a significant difference between the groups after the 2-week ingestion (P=0.015). Among the secondary endpoints, cytokines and chemokines levels involved in activating innate immunity maintained or enhanced only in the yogurt group. ALT, LDH, and CK significantly elevated only in the placebo group. Furthermore, amino acid levels were significantly lower in the placebo group after ingestion than that at baseline; however, it remained unchanged during the pre-exercise level in the yogurt group. Consuming yogurt fermented with OLL1073R-1 prevents the decline in immune function associated with strenuous exercise. Additionally, the yogurt may contribute to stable physical condition.