AbstractProtein tyrosine phosphorylation may be an important indicator of both the proliferative status and differentiation status of cells during embryonic development. To determine how each of these factors contributes to the level of phosphotyrosine-containing proteins detectable in embryonic tissues we have used immunohistochemistry with anti-phosphotyrosine antibodies on sections of developing chicken embryos. In contrast to an earlier study (Takata and Singer, 1988) we found proteins phosphorylated on tyrosine residues to be present in many dfferent cells of the developing chicken embryo. The successful detection of phosphotyrosine-containing proteins in many cell types required the presence of sodium orthovanadate, a phosphotyrosine phosphatase inhibitor, during fixation. Despite the fact that the majority of tyrosine kinases identified to date are growth factor receptors, the highest levels of phosphotyrosine-containing proteins in many tissues were localized to populations of cells which were differentiating or migrating rather than dividing.