Indirect immunofluorescent staining of CR1 versus serotonin in RBL-CR1. Cells were grown on glass coverslips, fixed, and saponin permeabilized and stained using anti-CR1 and FITC-antimouse immunoglobulin (A) or antiserotonin YMC1019 (rat monoclonal antiserotonin) followed by FITC-antirat immunoglobulin (B).