The human cell surface antigen CD38 is a 46-kDa type II transmembrane glycoprotein with a short N-terminal cytoplamic domain and a long Cys-rich C-terminal extracellular one. Although CD38 and Aplysia ADP-ribosyl cyclase exhibit amino acid-sequence homology and both catalyze the formation of nicotinamide from β-NAD⁺, the other reaction products of the two enzymes are different; ADP-ribose and cyclic ADP-ribose being predominantly generated by CD38 and Aplysia cyclase, respectively. To identify the structural determinant responsible for the different reaction products, we synthesized a series of chimeric proteins between the two enzymes in COS-7 cells. Our results indicated that a limited sequence near the carboxyl terminus of CD38 plays an important role not only in the enzyme activity but also in determination of the enzyme type categorized as a hydrolase. We also investigated the intracellular signaling mediated through CD38 in HL-60 cells which had been caused to differentiate by retinoic acid (RA). Addition of an anti-CD38 monoclonal antibody (mAb) to the cells induced a rapid tyrosine phosphorylation of the cellular proteins with the molecular weights of 120, 000, 87, 000 and 77, 000. Tyrosine kinase activity in the cell lysate immunoprecipitated with anti-phosphotyrosine mAb was also stimulated after the addition of the anti-CD38 mAb. Moreover, one of the prominent phosphorylated proteins stimulated by the anti-CD38 mAb was identified as the cbl proto-oncogene product, p120^c-cbl. These results indicated that tyrosine phosphorylation of cellular proteins including pl20^c-cbl is possibly involved in transmembrane signaling mediated through CD38.