Elevated brain lactate has been observed by in vivo proton MRS in different pathological situations. The origin of this lactate remains controversial. The possibility that it was produced by the metabolism of phagocytic cells has been proposed. To investigate this hypothesis, the authors have employed high-resolution proton MRS to monitor changes in glucose, lactate, and other metabolites in the medium used to culture human monocyte-derived macrophages in vitro. Results show that the differentiation of human monocytes/macrophages in the presence of physiological stimulating factors (M-CSF or GM-CSF) was associated with an increase in lactate production and glucose utilization. The present results are consistent with the hypothesis that lactate detected by proton MRS in vivo may be produced by the metabolism of macrophages when infiltrates of these cells are present. The possible extrapolation of the authors' finding to the in vivo situation and its relevance are discussed.