Cells of the immune system synthesize, store, and secrete polypeptide and amino acid type hormones, which also influence their functions, having receptors for different hormones. In the present experiment immunophenotyped immune cells isolated from bone marrow, thymus, and peritoneal fluid of mice were used for demonstrating the adrenocorticotropic hormone (ACTH) and triiodothyronine (T3) hormone production of differentiating immune cells. Both hormones were found in each cell type, and in each maturation state, which means that all cells are participating in the hormonal function of the immune system. The lineage-independent presence of ACTH and T3 in differentiating hematopoietic cells denotes that their expression ubiquitous during lymphocyte development. Higher ACTH and T3 content of B cells shows that these cells are the most hormonally active and suggests that the hormones may have an autocrine regulatory role in B cell development. Developing T cells showed heterogeneous hormone production which was associated with their maturation state. Differences in the hormone contents of immune cells isolated from different organs indicate that their hormone production is defined by their differentiation or maturation state, however, possibly also by the local microenvironment.