Ionophore-activation of RBL-CR1 results in increased cell surface expression of CR1. Cells were incubated for 60 minutes at 37°C in HBBS/g containing l μmol/L A23187 to which 1.2 mmol/L CaCl₂ was added (Ca2⁺). Controls included: cells incubated in HBSS without A23187 (none); only A23187, without CaCl₂ (alone); and A23187 with CaCl₂ plus 5 mmol/L EDTA. Cells were stained as described and the mean fluorescence of 5,000 cells from each condition was determined by FACS analysis. Results are the mean ± standard error of mean (SEM) for four to six experiments.