Cultures of human CD34pos cells stimulated with erythroid growth factors plus dexamethasone, a model for stress erythropoiesis, generate numerous erythroid cells plus a few macrophages (approx. 3%; 3:1 positive and negative for CD169). Interactions occurring between erythroblasts and macrophages in these cultures and the biological effects associated with these interactions were documented by live phase-contrast videomicroscopy. Macrophages expressed high motility interacting with hundreds/thousands of erythroblasts per hour. CD169pos macrophages established multiple rapid ‘loose’ interactions with proerythroblasts leading to formation of transient erythroblas-tic island-like structures. By contrast, CD169neg macrophages established ‘tight’ interactions with mature erythrob-lasts and phagocytosed these cells. ‘Loose’ interactions of CD169pos macrophages were associated with proery-throblast cytokinesis (the M phase of the cell cycle) suggesting that these interactions may promote proerythrob-last duplication. This hypothesis was tested by experiments that showed that as few as 103 macrophages significantly increased levels of 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide incorporation frequency in S/G2/M and cytokinesis expressed by proerythroblasts over 24 h of culture. These effects were observed also when macrophages were co-cultured with dexamethasone directly conjugated to a macrophage-specific CD163 antibody. In conclusion, in addition to promoting proerythroblast proliferation directly, dexamethasone stimulates expansion of these cells indirectly by stimulating maturation and cytokinesis supporting activity of macrophages.