We have recently purified from bovine brain a 19-kDa protein, p19, that was previously shown to undergo hormonally regulated phosphorylation in several neuroendocrine tumor cells. We now report the tissue distribution of p19, studied by immunoblotting. Using a rabbit antiserum, which binds both to the unphosphorylated form and to the two predominant phosphoforms of p19, we show that the protein is present in brain and testis but not in a variety of other mammalian tissues. High levels of p19 are also present in several cultured tumor cells expressing neuroendocrine properties. In addition, p19 was detected in HL60 promyelocytic leukemia and in Friend erythroleukemia cells, but not in several other cell lines. In rat brain, we show that the level of p19 is maximal on the first postnatal day and declines within the first 2 weeks of life to a low plateau that persists into adulthood. The concentration of translatable p19 mRNA also decreases postnatally in rat brain, suggesting that the developmental regulation of the expression of p19 occurs, at least in part, at a pretranslational level. The broad species cross-reactivity of the p19 antibody suggests that the gene encoding p19 has been highly conserved during mammalian evolution. Based on the pattern of expression of this protein, we propose that p19 plays a role in the development of neurons and neuroendocrine cell types.