Human milk was investigated in regard to the presence of lymphocyte growth factors. Precipitated and dialyzed protein from mature human milk was found to exhibit interleukin-1 (IL-1) activity in the murine thymocyte proliferation assay. A mitogenic effect of milk protein was also observed in cultures of human and guinea pig thymocytes, demonstrating that the effect was not species restricted. No interleukin-2 or thymocyte growth peptide activity was found. Gel filtration chromatography on Sephadex G 150 revealed three distinct peaks of IL-1 activity, with apparent molecular weights of 14, 31 and 60 kilodaltons, respectively. Chromatofocusing showed IL-1 activity in four peaks, the major peak eluting at an isoelectric point of 5.0. Similar molecular heterogeneity has previously been demonstrated for IL-1 isolated from macrophages and other sources. The physiological role of milk-derived IL-1 is unknown, but it is proposed that it might act as an immunostimulating factor in the mammary gland, and hypothetically also in the breast-fed infant. Since IL-1 is a potent pyrogen, it is also suggested that milk-derived IL-1 contributes to the well-known pyrogenic activity of milk.