The possibilities of combined radioprotection, using preirradiation WR-2721 administration and post- or preirradiation Broncho-Vaxom administration in lethally whole-body gamma-irradiated mice were investigated. LD50/30 dose reduction factors (DRFs) for mice treated with WR-2721 (200 mg/kg i.p. 30 min before irradiation), Broncho-Vaxom (25 mg/kg i.p. 24 h before irradiation), or both agents were 1.92, 1.17 and 2.07, respectively. These results demonstrated at least additive radioprotective effects of both agents, manifested in increased survival of irradiated mice. Radioprotection from 17 Gy was optimal when WR-2721 in combination with Broncho-Vaxom was given 30 min before irradiation and Broncho-Vaxom 24 h before or 4-8 h after irradiation. Combined modality treatments were also more effective than individual treatments alone in accelerating the bone marrow GM-CFC restoration. During the first days after irradiation enhanced colony-stimulating activity (CSA) of the lungs was observed in mice with postirradiation injection of Broncho-Vaxom alone or in mice injected with WR-2721 and Broncho-Vaxom (8 h after irradiation), as well as in mice only irradiated.