[en] The efficiency of haemopoietic cell transplantation into lethally irradiated mice is known to depend upon the degree of histocompatibility between the donor and the host and on the number of transplanted cells. With the extension of the time interval between irradiation and transplantation a proportionally higher number of transplanted haemopoietic cells was required to maintain the curative effect of transplantation. The haemopoietic microenvironment in lethally exposed mice remained unchanged until the tenth day after irradiation. (author)