In this study we tested the stimulatory effect of adamantylamide-l-alanyl-d-isoglutamine (AdDP) or its liposomal formulation (L-AdDP) on recovery of the granulocyte-macrophage hemopoietic progenitor cells in the bone marrow of sublethally irradiated mice of various ages. Number of GM-CFC progenitors in femur on day 10 was used as a parameter reflecting the stimulatory activity. Mice (aged 3-5 month) pre-treated with AdDP or L-AdDP via s.c. route displayed enhanced recovery of the granulocyte-macrophage hemopoietic progenitor cells at the dose of 5.5 Gy. Overaged mice (2 years) responded to the treatment when the dose was increased to 6.5 Gy, while radiation doses below 5.5 Gy should be used to see the stimulation effect in young mice (6 weeks). Entrapment of AdDP into liposomes enhanced costimulatory activity of sera of treated mice and prolongated this activity at least for 30 h after stimulation, in comparison to the mice treated with free AdDP where the costimulatory activity was spanned only up to 12 h. In conclusion, L-AdDP represents a suitable formulation of AdDP that induced recovery of GM-CFC progenitors in the femur of irradiated mice of various ages. The stimulatory effect depends on the extent of injury to bone marrow hemopoietic microenvironments caused by various doses of gamma-irradiation.