Hematopoietic patterns have been assessed in chronic 60Co gamma irradiated dogs during preclinical phases of evolving aplastic anemia (AA) or myeloproliferative disease (MPD), principally myeloid leukemia. Within the AA-prone dog, a singular phase of progressive decline in blood levels of granulocytes and monocytes was noted along with a similar reduction in marrow progenitors committed to granulocyte/monocyte differentiation (CFU-GM). Measured radioresistance of the preAA CFU-GM in vitro, relative to control CFU-GM from nonirradiated animals, revealed only slightly increased resistance to gamma rays, but significantly increased resistance to fission neutrons. Within the MPD-prone dogs, four preclinical phases (i.e. suppression, partial recovery, accommodation, and preleukemic transition) preceding development of overt MPD were evidenced by the monitored change in blood granulocyte/monocyte counts and marrow progenitor levels. Analysis of radioresistance of preMPD CFU-GM revealed marked changes with time of exposure and, in turn, with preclinical phase transitions. Gamma ray resistance increased in the initial phases of exposure, with maximal levels occurring during the middle phase of exposure (accommodation, phase III) followed by a tailing off of resistance at later times. Resistance to fission neutrons by preMPD CFU-GM was observed as well, but somewhat later in the exposure course and at a much lower, more consistent level. These differential patterns of radioresistance expressed by marrow CFU-GM of chronically irradiated MPD-prone dogs to gamma rays and fission neutrons gave rise to preclinical phase-specific 'relative biological effectiveness' (RBE) values. From these observations, we conclude that: (i) CFU-GM of MPD-prone dogs acquire and maintain marked radioresistance to low linear energy transfer (LET) gamma rays, but only marginally elevated radioresistance to high-LET fission neutrons during the course of chronic gamma ray exposure; and (ii) CFU-GM of the AA-prone dog, in contrast, acquire little change in resistance to gamma rays, but, surprisingly, marked resistance to neutrons relative to progenitors from nonirradiated controls. These results support the concept that acquired radioresistance of vital granulocyte/monocyte lineage-committed hematopoietic progenitors is temporally, perhaps causally, linked to the processes mediating hematopoietic recovery and accommodation under chronic irradiation, and in turn to preclinical events of evolving MPD. In addition, the marked differential responses of progenitors to gamma and neutron irradiation in vitro might suggest differences in the nature of cellular lesions elicited by chronic gamma irradiation, in vivo.