Abstract Dipeptides of the EW-sequence, consisting of d-amino acids, were shown to inhibit spleen colony formation both after in vitro treatment of bone marrow and after the peptide injection to the donor 2–48 h prior to bone marrow taking. The inhibiting effect of d-(EW) peptides can be eliminated by injection of thymic cells to the recipient. l-(EW) peptides have no influence on colony forming activity of intact bone marrow cells but stimulate regeneration of colony formation by irradiated (1 Gy) bone marrow. Unlike l-(EW), the d-(EW) peptides suppress the development of hemopoietic stem cells into the S-phase. Supposedly, d-(EW) represent a new generation of immunoactive peptides possessing of inhibiting activity.