Circulating levels of Mpl ligand in healthy human volunteers after bolus subcutaneous dosing of study drug. Serum concentrations of Mpl ligand were determined daily for 28 days in 16 normal human subjects using immunogenic ELISA assay after bolus subcutaneous injections of study drug. Subjects were randomized to receive PEG-rHuMGDF (3 μg/kg), or placebo in a 3:1 ratio, respectively. The results in 12 subjects receiving active drug are shown in the solid circles. The data for the 4 placebo subjects are represented by the solid squares. Peak values of 500 ng/mL developed on the second day after drug administration. The interval required for drug levels to decrease by 50% was 53 hours. The variance about the mean values represents ± 1 standard deviation (± 1 SD).