The presence of interleukin-12 (IL-12) in 39 samples of human milk was investigated, using the enzyme-linked immunosorbent assay. IL-12 (>40 pg/mL) was detected in 24 of the 39 samples collected (1408 +/- 2256 pg/mL, mean ? SD, ra = 24). A range of concentrations of IL-12 was observed in colostrum, transitional, and mature milk, with an apparent decrease in the mean concentration over time postpartum.