Using a semi-quantitative polymerase chain reaction (PCR) technique we have examined the expression of a panel of cytokines during thymus development, localizing the expression to individual components of the thymic stroma and thymocytes at different maturational stages. The expression of interleukin (IL)-7, stem cell factor (SCF), IL-1α and granulocyte-monocyte-colony-stimulating factor (GM-CSF) mRNA was mapped to individual stromal cell types, while the expression of IL-1α and GM-CSF, along with interferon (IFN)-γ and IL-4 was detected in the lymphoid compartment of fetal day (Fd) 14 thymus. The expression of lymphoid-specific cytokine genes was selectively down-regulated in thymocytes undergoing maturation. CD3−/104+8+ cells, representing an intermediate stage of thymocyte maturation, were devoid of cytokine gene expression. Their CD3+ progeny, on the other hand, expressed IFN-γ mRNA, supporting the notion that positive selection of cells for further maturation induces the reexpression of some cytokine genes. The cytokine profiles of the various stromal components differed. Purified major histocompatibility complex class II+ cortical epithelial cells strongly expressed IL-7 and SCF, but only limited expression of IL-lα and GM-CSF could be detected. Fetal mesenchyme, on the other hand, expressed SCF, IL-lα and GM-CSF but not IL-7. The importance of these cytokine profiles in relation to T cell development is discussed.