Effects of increasing concentrations of L-arginine on ZL-treated erythroid progenitors isolated from patients with homozygous sickle cell anemia. Cells were cultured on methylcellulose culture media and treated with ZL (100 μM) as described in “Patients, materials, and methods.” L-arginine (10-100 μM) was added at the beginning of the incubation period. The number of BFU-E colonies (▪) and levels of hemoglobin F (Hb F; ▾) were measured after 14 days of incubation. Data are expressed as mean ± SEM of 4 experiments done in triplicate. *P is less than .001 compared with baseline (BL); BL indicates the number of BFU-E colonies or percent Hb F measured in cultured cells not treated with ZL. The 0 μM point indicates the number of BFU-E colonies or percent Hb F measured in cultured cells treated with ZL alone. θ indicates that P is less than .01 c/w L-arginine (0 μM); ψ, P is less than .05 c/w L-arginine (0 μM); and ϕ, P is less than .001 c/w L-arginine (0 μM).