Effects of L-arginine and L-NMMA on HU-treated erythroid progenitors isolated from patients with homozygous sickle cell anemia. The experimental conditions were the same as in Figure 3. L-arginine (ARG; 100 μM) effects on hydroxyurea (HU)–treated (100 μM) erythroid progenitors in the absence and presence of the nitric oxide synthase inhibitor, N^G-monomethyl–L-arginine (L-NMMA; 100 μM) were assessed. The number of BFU-E colonies (A) and percent Hb F (B) were measured as described in “Patients, materials, and methods.” Data are expressed as mean ± SEM. n = 4 in each group. Each experiment was done in triplicate. *P is less than .001 compared with control (CON).