The present invention relates to a proteinaceous extract derived from tortoise spleen and to a tetrapeptide FTGN, which have stimulatory activity on hematopoietic cells. In particular, this tetrapeptide enhances hemopoietic reconstruction, and bone marrow re-population, reduced as a consequence of a high dose of radiation or chemotherapy exposure. The invention further provides pharmaceutical compositions comprising as an effective ingredient the proteinaceous extract or the FTGN tetrapeptide and ex vivo and in vivo methods of treatment employing them.