We have evidence that thymic regulatory cells can either enhance or inhibit the growth of hematopoietic progenitors in vitro. We have suggested that two separate populations are responsible for this regulatory interaction but isolation of the cell types has proven difficult. We now report the isolation by counterflow centrifugal elutriation (CCE) of two separate populations of thymocytes which regulate erythropoiesis in coculture. We demonstrate that a minority population (less than 10%) of slow sedimenting elutriated thymocytes provide a helper function whereas the suppressor population is the majority population. Furthermore, some thymocytes of intermediate cell volume neither enhance nor inhibit erythroid colony growth. We conclude that isolation of thymic subsets can lead to identification of populations which induce cell-cell regulation of hematopoietic progenitors resulting in both a positive and negative feedback control of growth.