We have examined a possible role of two different types of irradiated stromal cells, i.e. murine bone marrow (BM) stromal cells and stromal cell line MS-5R, when cocultured with murine blood-borne progenitors or sorted Lin- Sca-1+ bone marrow cells in vivo in peritoneal diffusion chambers (DC). Retrieval and quantification of the cultured cells were performed after 4, 7, and 14 d. Granulocyte and/or macrophage colony-forming cells (G/M-CFC) were enumerated in subcultures from the DC. G/M-CFC production was not enhanced in the stroma-contact cultures, in comparison with the standard stroma-non-contact cultures, but early granulocytopoiesis was stimulated. Perturbation of the humoral environment of DC was investigated in a number of ways, for example with continuous infusion of rhG-CSF from a subcutaneous implanted minipump to DC host mice, with DC host mice carrying a transplantable leukaemia, secreting interleukin 3 (IL-3), and with injections of various cytokines. None of these interventions sustained the expansion of the G/M-CFC population. In conclusion, for ectopic haematopoiesis to take place, several requirements must be met. Relevant stromal cells apparently affect haematopoiesis both via direct cell-cell interactions and via humoral mediators (viz. cytokines) which they secrete.