The enhanced proliferation of erythroblasts in cocultures requires direct contact with macrophages and is unrelated to FVA infection. (A) Effect of removing adherent erythroblasts from macrophages on their fold increase in subsequent culture. indicates adherent erythroblasts cultured in cocultures; , control erythroblasts cultured alone; , adherent erythroblasts removed from macrophages and recultured alone. Adherent erythroblast accumulations in coculture were significantly greater at all times than control erythroblasts or formerly adherent erythroblasts that were cultured alone (*P < .002). (B) Effect of macrophage-conditioned medium at various concentrations (○, 0%; ◇, 8%; ◀, 16%; ▶, 30%; □, 60%) on fold increase in total erythroblasts cultured alone; erythroblasts cocultured with macrophages (■). Erythroblasts in cocultures accumulated to significantly greater numbers than in any concentration of conditioned medium (*P < .05). (C) Fold increase in splenic erythroblasts of bled mice, cocultured with splenic macrophages (■), and cultured alone as controls (○). Data are means plus or minus SE of 3 separate experiments for each part; no significant increases above controls were found for formerly adherent erythroblasts cultured alone (P < .1-.47) or erythroblasts cultured with any concentration of macrophage-conditioned medium (P < .25-.5), whereas erythroblasts from bled mice were always increased significantly in cocultures compared with controls (*P < .004-.4).