Transgenic zebrafish embryos expressing tissue-specific green fluorescent protein (GFP) can provide an unlimited supply of primary embryonic cells. Agents that promote the differentiation of these cells may be beneficial for therapeutics. We report a high-throughput approach for screening small molecules that regulate cell differentiation using lineage-specific GFP transgenic zebrafish embryonic cells. After validating several known regulators of the differentiation of endothelial and other cell types, we performed a screen for proangiogenic molecules using undifferentiated primary cells from flk1-GFP transgenic zebrafish embryos. Cells were grown in 384-well plates with 12,128 individual small molecules, and GFP expression was analyzed by means of an automated imaging system, which allowed us to screen thousands of compounds weekly. As a result, 23 molecules were confirmed to enhance angiogenesis, and 11 of them were validated to promote the proliferation of mammalian human umbilical vascular endothelial cells and induce Flk1+ cells from murine embryonic stem cells. We demonstrated the general applicability of this strategy by analyzing additional cell lineages using zebrafish expressing GFP in pancreatic, cardiac, and dopaminergic cells.