During embryonic development, the components of the avian immune system undergo ontogeny in several distinct organs, including the bone marrow, spleen, thymus, and bursa of Fabricius. This process is regulated and controlled by the complex interactions of various cytokines and colony-stimulating factors (CSF). The objective was to examine the action of two different sources of hematopoietic growth factors, spleen-conditioned media (SCM) and chick embryo extract (CEE), on the proliferation of hematopoietic cells from various organs and on the differentiation of progenitor cells in semi-solid culture. Spleen and bone marrow cells obtained at Day 16 of incubation responded in a dose-dependent manner to the addition of SCM and CEE alone or in combination. No proliferative effect of SCM was observed on cells obtained from embryonic thymus or bursa. Clonal analysis of bone marrow and spleen cells suggested that CEE may contain the avian equivalents of stem cell factor, interleukin-3, granulocyte-macrophage CSF, granulocyte-CSF, and macrophage-CSF. Clonal analysis of SCM cultures suggested that in addition to myelomonocytic growth factor, which affects primarily macrophage-granulocyte lineages, a thrombocyte-CSF-like activity was also apparent. The SCM alone tended to act upon committed late progenitors. The combination of CEE and SCM amplified the size and the total number of colonies obtained and appeared to act synergistically upon progenitors with a high level of proliferative potential. This response on young progenitors was confirmed when cells were cultured in CEE and SCM prior to clonal analysis. These results document the presence of thrombocyte CSF in SCM and the effect of both CEE and SCM on the proliferative differentiation of avian embryonic hematopoietic progenitors.