Effect of NAC on SS erythrocyte density. Low density SS erythrocytes that were incubated at 37°C for 16 hours without cycling or NAC (A), with cycling, but without NAC (B), and with cycling in the presence of 20 mmol/L NAC (C), were placed on Percoll gradients (45% to 65%) and centrifuged at 907g for 45 minutes. Note NAC's ability to inhibit dense cell formation. Density of the RBCs in different Percoll layers was obtained by sedimenting density marker beads (Pharmacia, Piscataway, NJ) under identical conditions.