Sequential ¹¹¹In-labeled platelet disappearance curves in healthy human volunteers receiving PEG-rHuMGDF (3 μg/kg). Disappearance curves are depicted for¹¹¹In-labeled platelets in 8 normal subjects at baseline (solid circles), in 6 subjects having received bolus subcutaneous injections of PEG-rHuMGDF (3 μg/kg) 8 days previously (solid squares), 12 days previously (solid triangles), and 18 days previously (solid diamonds). Platelet populations enriched with newly formed platelets on day 8 demonstrate upward bowing of the disappearance curve with a computed life span of 226 ± 22 hours. Conversely, platelet populations enriched with platelets approaching senescence on day 18 show accentuated downward bowing of the disappearance curve with a computed life span of 178 ± 53 hours. The differences are statistically significant between baseline and 8-day determinations and day 8 and day 18 values. The disappearance pattern of day 12 platelets resembles the baseline pattern. The placebo subjects yielded results consistent with baseline values. Variance about the mean values is ± 1 SD.