The improvement of techniques for in vitro expansion of cord blood (CB) hematopoietic stem cells (SCs) is, at present, one main task of tissue engineering. Hence, we investigated whether endothelin-1 (ET-1) and adrenomedullin (AM), two regulatory peptides exerting growth promoting action on several cell systems, favor the in vitro expansion of CB SCs in liquid culture. CB hematopoietic cell middle-term expansion was carried out in a stroma-free liquid culture medium in the presence of ET-1, AM and three different cytokine combinations. After two weeks of incubation, aliquots of expanded-cell suspension were seeded on semisolid medium and clonogenic tests were carried out by counting the number of colony forming units (CFUs) after 14 days of culture. Neither ET-1 nor AM (2.5 x 10(-8) M) were per se able to significantly increase the CFU number, but both peptides magnified the pro-expansive effects of some cytokine cocktails. In light of these findings, we conclude that ET-1 and AM are to be considered novel promising molecules that, in association with cytokines, can be utilized as pro-expansive factors of CB SCs in prevision of their clinical use in allogeneic transplantation.