This study was undertaken to analyze postpartum changes in concentrations of interleukin (IL)-10 and IL-12 through the 3 stages of lactation. A total of 87 human milk samples were collected from 29 healthy mothers during the colostrum (0-3 days), early milk (14-17 days), and mature milk (44-47 days) phases. Enzyme-linked immunosorbent assay tests were performed on the milk samples. IL-10 was detected in 7 and IL-12 in 4 of the colostrum samples. in the transitional milk samples, IL-10 was present in 4 and IL-12 in 2; however, both of these cytokines became undetectable in mature milk samples. The decrease in concentrations of IL-10 and IL-12 was statistically significant during the postpartum period (P=.001 and P=.024, respectively). IL-10 levels in the colostrum samples were higher than in the transitional samples (P=.018, with use of the post hoc test). No statistically significant differences between IL-12 levels were noted in the colostrum samples and the transitional samples (P=.068, with use of the post hoc test). A negative correlation was observed between concentrations of IL-10 in colostrum and the total number of pregnancies (R=-.401; P=.031). The findings of the present study suggest that mean concentrations of IL-10 and IL-12 are decreased in human milk as lactation continues through its 3 phases.