Data extracted from the γ Beagle Dog Tissue Archive hosted by the Woloschak Laboratory (Chicago, IL, USA) were used in a retrospective comparative study of the influence of associated morbidities on the ability of organisms to regenerate hematopoietic cells. There were 209 dogs that had been subjected to whole-body irradiation from a cobalt 60 source from 1.1 years of age until death, with dose rates of 0.003-0.038 Gy/day. One hundred fifty-six nonirradiated dogs served as controls. The presence of neoplasms clinically recorded during each dog's lifetime (uncertain benign or malignant nature) was used to assign experimental subjects and controls to one of two groups: W (with) and WO (without) lesions. Differences between the two groups were noted, mainly a longer average life span and a prevalence of solid malignancy over malignant hematopoiesis in both irradiated and nonirradiated W dogs. We propose an interpretation of these data as showing variability of the activity of tissue-committed hematopoietic stem cells, reflected not only by an increased incidence of solid benign and malignant tumors, but also by greater marrow radioresistance and a stronger viability of W cohorts.