Effects of L-arginine and L-NMMA on zileuton-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 zileuton-treated erythroid progenitors in the absence and presence of the nitric oxide synthase inhibitor LNMMA (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 = 7, control (CON); n = 7, zileuton (ZL)–treated; n = 5, ZL + L-arginine (ARG); and n = 4, ZL + ARG + LNMMA–treated erythroid progenitors. Each experiment was done in triplicate. *P is less than .001 compared with CON and ZL + ARG.