Xenopus and murine activin A homologues (XTC-MIF and WEHI-MIF) and Xenopus and bovine basic fibroblast growth factor (bFGFs) are potent inducers of mesodermal and endodermal pathways of development in amphibian blastular animal cap cells. Porcine transforming growth factor beta 2 (TGF beta 2) is a weaker inducer in the same assay but human platelet-derived growth factor (PDGF) is inactive. We have assayed these factors for evidence of homologous effects in bird development. Unlike amphibians, bird embryos never exhibit a clean segregation of a cell layer that has a uniform specification when uninduced, and can be cultured in isolation as an assay after exposure to soluble factors. We have therefore performed less direct experiments, of three types. We have briefly cultured early chick epiblast cells with and without factors and then assayed their capacity to attach and spread upon fibronectin, in comparison with young streak and substreak hypoblast cells. We have asked whether similar microculture with factors alters the ability of quail epiblast cells to disrupt morphogenesis, and to integrate into the structure, of host chick blastoderms into which they are seeded. Finally, whole early chick blastoderms have been preincubated with or without factors for a brief period before setting them up to develop in vitro under circumstances usually permitting successful formation of axial pattern. Strong effects of the activin-like factors, of bFGF and of TGF beta 2 were seen in all three procedures, while PDGF was essentially inactive. In epiblast cells, effective factors at picomolar concentrations induced stable spreading upon fibronectin, and a capacity to adhere and spread upon basal epiblast surface and prevent morphogenesis in host blastoderms. Preincubation of whole early blastoderms with these factors led to characteristic deviation from normal development over the subsequent 24 h. We therefore suggest that peptides from the particular families that are active as inducers in amphibian blastula ectoderm may mediate homologous or closely related steps in respecification throughout vertebrates.