A technic for the in vitro labeling of human platelets with DFP32 is presented, critically evaluated, and compared to in vivo methods employing DFP32 and to in vitro methods using Cr51. The initial recovery of platelets labeled in vitro with DFP32 averaged 79 per cent, but the survival curve was characterized by an irreversible initial loss of platelet radioactivity. Experiments in which platelets were simultaneously labeled in vitro with both DFP32 and Cr51 suggest that this is not due to elution of DFP32. The survival curve of platelets labeled in vivo with DFP32 shows an initial transient reduction in platelet radioactivity. It is suggested that both of these aberrations in initial survival are the result of platelet injury by DFP32. Significant "tailing" was observed in the survival curves obtained with DFP32, and possible explanations of this phenomenon are discussed. DFP32-labeled platelets circulating after 5 hours apparently survive normally and disappear from the circulation as a rectilinear function over the next 6-8 days. Although both in vitro and in vivo labeling methods employing DFP32 provide a meaningful approximation of platelet lifespan, the initial and terminal aberrations of the survival curves greatly complicate further interpretation. Dextran had no detectable effect on platelet survival, and epinephrine, Mecholyl, and cutaneous vasodilatation did not alter the platelet count or the specific activity of circulating labeling platelets in human subjects. The problem of initial platelet survival and the question of an extravascular or marginal platelet pool is discussed in the light of these data.

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