Virtually pure primary cultures of normal mammary epithelial cells (MEC) obtained from healthy women were shown to release interleukin 6 and 8 (IL6, IL8) and to produce a nonsecreted form of tumor-necrosis factor (TNF). No interferon (IFN), whether α, β or γ, or ILI-α or -β could be detected. Analysis of cellular RNA confirmed these findings and showed that MEC also express IL6 receptor and TNF-α-related mRMAs. Epithelial cells were selectively stained by antibodies to IL6, IL8 and TNF-α both in primary cultures and in the normal mammary gland. Samples of human milk contained sizable amounts of IL6, IL8 and IFN-γ; yet the liquid phase was consistently negative for other cytokines (i.e., TNF-α, IFN-α/-β, ILI-α1 infiltrating carcinomas and greatly reduced in cultures of oncogene-transfected mammary cells, suggesting that alterations of IL6 expression are associated with pathogenesis in breast cancer.