RTP, also called Drg1/Cap43/rit42/TDD5/Ndr1, was originally identified as a homocysteine-responsive gene product, and is now considered to be involved in stress responses, atherosclerosis, carcinogenesis, differentiation, androgen responses, hypoxia, and N-myc pathways. We raised an antiserum against a recombinant human RTP. Western blot analysis showed that RTP expression was induced in human umbilical vein endothelial cells under conditions causing endoplasmic reticulum stress. RTP was partially phosphorylated at seven or more sites. The phosphorylation was reversible, and was enhanced by an increased level of intracellular cAMP and inhibited by both a protein kinase A inhibitor and a calmodulin kinase inhibitor. Protein kinase A directly phosphorylated recombinant RTP in vitro. The phosphorylated forms were abundant in cells at the early log phase, and then decreased with increasing cell density. These data demonstrated that RTP is a phosphorylated stress-responsive protein, and its phosphorylation may be related to cell growth.