Circadian differences in the susceptibility of the marrow to the effects of radiation, myelotoxic drugs, and growth factors suggest that hematopoietic processes vary significant throughout each day. One mechanism possibly responsible for the differing degrees of marrow damage sustained from a fixed dose of a cytotoxic agent at different times of day is the circadian organization of cell cycle events. Previous circadian rhythm-oriented studies of proliferation using unfractionated marrow have reported seemingly contradictory peak and nadir times of day. Marrow represents a heterogeneous population of stem cells and various hematopoietic progenitors whose proliferation and differentiation are controlled by both common and unique factors. Therefore, we examined lineage-specific circadian marrow proliferative dynamics for evidence of parasynchronous circadian DNA synthesis. Cell cycle phase was determined using flow cytometry with both propidium iodide staining and 5-bromo-2'-deoxyuridine (BrdU) incorporation concurrently with cell culture-based determinations of lineage-specific progenitor numbers in the same marrow samples. Although no clear circadian (24-hour) rhythm characterized unfractionated marrow DNA synthesis, both erythroid- and myeloid-enriched subpopulations demonstrated distinct circadian patterns with respect to the percentage of cells incorporating BrdU, with up to 50% differences throughout each day. Interestingly, these circadian rhythms in erythroid and myeloid progenitor cell DNA synthesis are entirely different from one another. The lineage-specific circadian patterns in the fraction of cells undergoing DNA synthesis are, in part, paralleled by up to eightfold larger circadian differences in erythroid and myeloid colony numbers. Multipotential colony numbers likewise vary throughout the day, with a unique pattern of their own. The predominant period length of daily rhythms in colony numbers and their amplitudes differ as a function of the stage of progenitor commitment. Multipotent and early progenitor colony numbers each exhibit 24-hour rhythms, with three- to fivefold daily peak-trough differences, whereas later progenitor colony numbers exhibit two peaks per day (12-hour rhythms) with twofold peak-trough differences throughout each day. In vivo erythropoietin (Epo) administration enhances daily rhythms in erythroid colony numbers by increasing their amplitudes while leaving their circadian shapes virtually unchanged. The increment in erythroid colony numbers after Epo administration varies up to 16-fold with the time of day of treatment. In summary, we have defined distinctly different lineage-dependent circadian patterns of marrow progenitor numbers and proliferating cells. We can infer from these data that the circadian timing of administration of physical, chemical, or biologic agents, whose bioactivity toward marrow precursors depends on the cell cycle phase of its presentation, can be expected to affect this activity predictably and significantly. These results may have practical applications in improving stem and progenitor cell yields by optimal circadian timing of growth factor administration and harvest.