Bone marrow samples from sternum and iliac crests were harvested every 4 hours during 19 24-hour periods from 16 healthy male volunteers, and myeloid progenitor cells were cultured by the colony-forming unit- granulocyte-macrophage (CFU-GM) assay. A large interindividual variation was observed in the mean number of colonies during each 24- hour period, with the highest 24-hour mean colony number being about 600% greater than the lowest (range: 16 +/- 2.3 to 100.3 +/- 4.5). For each individual the difference between the lowest and highest colony number throughout the day ranged from 47.4% to 256.3% of the mean colony number of each series. A circadian stage-dependent variation in the number of colony-forming units of myeloid progenitor cells (CFU-GM) of human bone marrow was demonstrated, with values 150% higher, on the average, during the day as compared with the night. The overall data (891 CFU-GM replicates) exhibited a significant 24-hour rhythm (P less than .001) with an acrophase at midday (12.09 hours with 95% confidence limits from 10.32 to 13.49 hours) and a trough at midnight. This 24- hour variation was found to covary with DNA synthesis in the total proliferating bone marrow cell population. A seasonal effect on CFU-GM numbers was detected by ANOVA (P = .014) and by the least squares fit of a 1-year cosine (P = .015), with the highest number found in summer. The potential relevance of these findings should be examined in relation to cytotoxic cancer therapy, use of hematopoietic growth factors, and bone marrow transplantation.