IRP activity in monocytes of controls, GH, and SH patients during in vitro maturation to macrophages. Monocytes from 10 controls (▧), 2 patients with SH caused by sporadic PTC and β-thalassemia intermedia (), 5 untreated (▪), and 6 treated unrelated GH patients (□) were allowed to differentiate in vitro. Lysates were prepared from cells freshly isolated (day 0) or cultured for 3 and 6 days and IRP activity was measured by RNA-bandshift assay as described in the legend to Fig 1. Values were calculated as described in Materials and Methods and are expressed as means with SD. The inset shows a representative bandshift analysis of samples from a control (A) and an untreated GH patient (B) at the various time points. IRP activity increased with differentiation to macrophages in all the populations examined, maintaining the differences observed in freshly isolated monocytes.