Recycling of US28. HeLa CD4-US28 cells were incubated with Q4120 antibody at 37°C for 1 h to allow antibody uptake, cooled on ice, and washed in acid medium to remove the Q4120 bound to the cell surface. (A) The cells were fixed, left intact or permeabilized with saponin (right and left, respectively), and then stained with a biotin-conjugated secondary antibody to detect Q4120 followed by fluorescein-streptavidin. (B) The cells were reincubated with a biotin-conjugated secondary antibody for 1 h at 37°C, cooled on ice, acid washed, fixed, and stained with fluorescein-streptavidin as in A. Scale bars, 10 μm. The righthand fields for each panel show Nomarski and fluorescence images of the same field.