Comparison of binding of three FR ligands and FR antiserum to KB and CD34⁺ cells. KB cells (□) and CD34⁺ cells (▨) were incubated with fluorescent FA-derivatized liposomes (FA-liposome; 4 hours at 37°C), FA labeled with fluorescein (FA-FITC; 1 hour at 37°C), ³H-FA (³H-FA; 2 hours at 37°C), or rabbit antiserum to placental FR (anti-FR; 30 minutes at 4°C). Fluorescent ligand binding to cells was quantitated by FACS, while ³H-FA binding was determined by scintillation counting. Relative uptake of fluorescent ligands was calculated from the product of average cell fluorescence intensity and percentage of positively gated cells. To display the number of ³H-FA molecules taken up per cell on the same axis as the fluorescent ligands, ³H-FA/cell values were divided by a factor of 20.