IgG binding to in vivo–aged biotinylated dog RBCs. Positive (○) and negative (•) RBCs were isolated at the indicated times post-biotinylation by magnetic cell sorting. Isolated RBCs were incubated with ¹²⁵I protein A and then washed five times before determining the number of protein A molecules per RBC. Each fraction was assayed in triplicate, and each point represents the mean ± 1 SD of two dogs. Day 0 data was obtained using RBCs collected just before the in vivo biotinylation.