The majority of platelets and monocytes are associated with PEG-Liposome. HemA mice received tail vein injection of PEG-Liposome or FITC-labeled PEG-Liposomes (FITC-PEG-Lip) either alone or formulated with 0.2 IU rFVIII (FITC-PEG-Lip/rFVIII). Fifty microliters of blood was sampled at 0.5, 24, and 48 hours and analyzed by flow cytometry. (A) Fluorescence-activated cell sorting profile on blood samples from a representative FITC-PEG-Lip–treated HemA mouse. Platelets were stained with fluorophore-conjugated monoclonal antibodies against mouse CD41 and CD61. A threshold for FITC fluorescence level was defined as below which fell 99% of all platelet events from blood injected with unlabeled PEG-Liposomes. (B) Percentage of CD41⁺, CD61⁺ platelets labeled by FITC-PEG-Liposomes in HemA mice treated with FITC-PEG-Lip alone or FITC-PEG-Lip formulated with rFVIII. Results presented are mean ± SD from 7 mice at each time point. (C) Fluorescence-activated cell sorting of blood from 1 FITC-PEG-Lip–treated HemA mouse, in which monocytes and lymphocytes were identified by characteristic light scatter profile. Blood from unlabeled PEG-Liposome–injected animals was used to determine FITC (FL1)–negative (x-axis) and no stain (FL4)–negative (y-axis) threshold of each cell type to correct for cell type–dependent autofluorescence. Upon FITC-PEG-Lip treatment, cell-associated FITC signal (x-axis) increased but unstained fluorescence signal (y-axis) remained constant.