Endo16 is a lineage-specific protein expressed during embryogenesis in the sea urchin, Strongylocentrotus purpuratus. We have examined the effects of various concentrations of lithium chloride, a well-known vegetalizing agent, on Endo16 expression in whole sea urchin embryos. Our results show that treatment with LiCl causes increased steady-state levels of Endo16 transcripts. An increase in the number of endodermal cells in treated embryos demonstrates a change in cell lineage. In addition, we observed a delay in development of lithium-treated embryos that is accompanied by a delay in the expression of a late class histone H2B gene.