Growing evidence shows that C1q modulates the growth and function of cells committed to the monocyte-derived dendritic cell (DC) lineage. Because C1q regulates both innate and acquired immune responses, we postulated that C1q modulates the transition from monocytes to DCs, i.e. the interface between innate and acquired immunity. Human peripheral blood monocytes cultured with soluble C1q and DC growth factors (granulocyte-macrophage colony-stimulating factor + Interleukin-4) failed to down-regulate monocyte-associated (CD14, CD16) and up-regulate DC-associated (CD83, CD86) markers. Impaired DC differentiation was not due to apoptosis; further analysis revealed the development of CD14(hi)CD11c(hi)CD16 (+/-) cells that have previously been associated with both innate and acquired immunity. Monocyte-DC precursors expressed gC1qR, the receptor for globular heads of C1q, from the outset, while cC1qR, the receptor for the collagen tails of C1q, was expressed at low levels. Notably, the binding pattern of monoclonal antibodies specific to the globular heads of C1q indicated that C1q is bound to monocytes via globular heads, presumably through gC1qR. Moreover, gC1qR levels decreased, while cC1qR levels were dramatically amplified as monocytes differentiated into immature DC. Thus, specific C1q/C1q receptor (R) interactions may control the transition from the monocyte state (innate immunity) toward the professional antigen-presenting cell state (adaptive immunity).