The growth of eosinophil progenitors (CFU-Eo) and its modulation by hydrocortisone (HC), mononuclear cells, interleukin 1 (IL-1), and interleukin 2 (IL-2) in three cases of eosinophilia are reported in this paper. One microM HC decreased the proportion of CFU-Eo in each of these three patients, and in each case, the addition of autologous mononuclear cells at a 1:1 ratio abrogated the effect of HC on CFU-Eo. As studied in two of the three cases, IL-1 and IL-2 were able to prevent the effect of HC. Further studies showed no effect of HC when monocyte T cell-depleted marrow cells were used as the target population. These results suggest that CFU-Eo production in eosinophilic states is subject to modulation by HC and T lymphocytes.