The expression of the Pit‐l/GHF‐1 transcription factor (hereafter Pit‐1), which controls the expression of growth hormone (GH) and prolactin (PRL) in the pituitary gland, has been documented in human and rat hemopoietic and lymphoid tissues and cell lines. Pit‐1 mRNA was detected by in situ hybridization in about 1 % of rat bone marrow cells and in the spleen red pulp and marginal zone. Pit‐1 was also expressed in human tonsils (mantle zone), in the thymus (rat and human, non‐lymphoid cells), in lipopolysaccharide‐stimulated rat peritoneal cells and in non‐hepatocyte cells in the liver (rat and human). A detailed investigation of the rat spleen showed a very similar distribution for Pit‐1, GH and PRL mRNA and Pit‐1, GH and PRL proteins (detected by immunocytochemistry). Using polymerase chain reaction followed by Southern hybridization, the expression of Pit‐1 could be confirmed in human and rat spleen, bone marrow and thymus. HL60 and RAJI leukemic cells were also positive. The sequence of fragments amplified from rat spleen and from human bone marrow completely matched published sequences of rat and human pituitary Pit‐1, respectively. Expression of GH and PRL in lymphoid tissues has been documented. The straightforward hypothesis would therefore be that Pit‐l's main function in lymphoid tissues is controlling GH and PRL expression, as in the pituitary gland. GH and PRL may be hemopoietic and lymphoid growth and differentiation factors.