Sir.—The report of Rubin and Rowley and the accompanying editorial concerning Hb S and HPFH in a newborn was of interest to us (Journal 1979;133:1248-1250 and 1235-1236). We have recently found a similar patient in our newborn screening program. However, we take exception to the statement that acid agar electrophoresis permits differentiation of sickle cell disease from Hb S/HPFH and Hb S/β thalassemia. The agar adds nothing to the differential diagnosis. As indicated by Rubin and Rowley, differentiation requires family study and/or other methods for establishment of the genetic basis for the Hb FS phenotype. We are also confused by the final comment in the editorial about the low incidence of sickle cell anemia in patients without Hb A.