Cellular accumulation of chloroquine. U-937 and U-A10 cells were exposed to chloroquine (0 to 50 μmol/L) for 2 hours before cells were washed and lysed and chloroquine was extracted. The amount of chloroquine was measured by HPLC analysis using 8-hydroxyquinoline as an internal standard. Shown is the mean (± the standard error) net cellular chloroquine accumulation when cells were exposed to 10 or 50 μmol/L chloroquine. The differences between U-937 and U-A10 cells were statistically significant at (▨) 10 (P < .005) or (▨) 50 μmol/L (P < .05) of chloroquine.