The cross-rcsistance induced by low-level radiation and low concentration of chemical mutagens was studied using human lymphocytes in vitro and mouse bone marrow cells and germ cells in vivo. The chemical mutagens used in these experiments included the MMC,HD, and CP. The results reported here, in addition to those that have appeared in the literature, show the following features documented for the first time: (1) MMC as an adaptive treatment could also induce the adaptive response to radiation-induced cytogenetic damage; (2) resistance against MMC-induced damage could be induced by low dose radiation in vivo; (3) it is in mouse germ cells that low dose radiation or chemical mutagens could induce the cross-adaptation; (4) the cells pretreated by low dose X-rays could show the high resistance to H2O2-induced cytogenetic damage; (5) CP as D1 treatment could not induce the adaptive response to radiation and low dose radiation also could not induce the adaptive response to CP The cross-rcsistance induced by low-level radiation and low concentration of chemical mutagens was studied using human lymphocytes in vitro and mouse bone marrow cells and germ cells in vivo. The chemical mutagens used in these assays include the MMC, HD, and CP. The results reported here, in addition to those that have appeared in the literature, show the following features documented for the first time: (1) MMC as an adaptive treatment could also induce the adaptive response to radiation-induced cytogenetic damage; (2) resistance against MMC-induced damage could be induced by low dose radiation in vivo; (3) it is in mouse germ cells that low dose radiation or chemical mutagens could induce the cross-adaptation; (4) the cells pretreated by low dose X- can could induce the high resistance to H2O2-induced cytogenetic damage; (5) CP as D1 treatment could not induce the adaptive response to radiation and low dose radiation also could not induce the adaptive response to CP