The neurohormone, melatonin, a product of the pineal gland, is a potent immune cell stimulant. Phytochemicals contained in root extracts of the plant species Echinacea purpurea are also potent as immune cell stimulants. Both agents are potent stimulants of T, B, and/or natural killer cells, but little is known of their effect on other hemopoietic cells, specifically granular leukocytes, also participants in a wide variety of disease defense processes. Given their current popularity and availability for amelioration of a) jet lag and sleep disorders (melatonin) and b) virus-mediated respiratory infections (E. purpurea), we investigated the effects of these agents on granular leukocytes and their precursors, myeloid cells. Mice received these agents daily for 7 or 14 days via the diet, thus mimicking human administration, after which spleens and bone marrow were removed and assessed for mature, differentiated granulocytes and their myeloid progenitors. The influence of these agents was directly related to the stage of cell maturity. Administration of both agents together resulted in significantly elevated levels of myeloid progenitor cells in both bone marrow and spleen and significantly reduced levels of mature, functional granulocyte progeny in both organs, suggesting a) increased precursor proliferation, b) antiapoptosis among the progenitors, and/or c) inhibition of precursor maturation-the latter readily explaining the paucity of mature granulocyte progeny. In conclusion, individual administration of either the herbal derivative and melatonin was either without effect (E. purpurea) or even advantageous (melatonin) to cells of this lineage, but when administered together, these agents significantly perturbed myelopoiesis.