Expression of CD38 on NK cells and CTLs. Contour diagrams of PBL incubated for 3 days in (A) medium alone, (B) medium containing IL-2, and (C) medium containing IL-2 plus 10 ng/mL actinomycin D. (D) Single-parameter histogram showing CD38 expression on CTLs. In (A) through (C), the NK (CD16⁺) cells were 95%, 93%, and 99% CD38⁺, respectively, and the mean CD38 fluorescence intensities on the CD16⁺CD38⁺ cells were 147, 134, and 232. In (D), cells were 90% positive for CD3 (not shown) and 80% positive for CD38.