Retinoic acid (RA) treatment of HL-60 cells induces surface expression of CD38. This lymphocytic antigen is also a novel bifunctional enzyme catalyzing the synthesis and hydrolysis of cyclic ADP-ribose (cADPR), a Ca2+ mobilizing metabolite of NAD+. The synthetic activity of CD38 is very difficult to detect because of the concurrent hydrolytic activity. In this study, a Ca2+ release assay capable of detecting submicromolar concentrations of cADPR was used to demonstrate the induction of ADP-ribosyl cyclase activity in HL-60 cells by RA. Concomitantly, cADPR hydrolase activity was also increased. The results were further substantiated by using a newly developed assay for GDP-ribosyl cyclase activity. This assay uses NGD+ as substrate instead of NAD+. The resulting fluorescent product, cyclic GDP-ribose, is resistant to hydrolysis and accumulates, making it a highly sensitive and convenient assay for CD38-like enzymes.