Survival of normal or M-CSF-exposed CMFDA labeled platelets in normal SW mice. SW mice received M-CSF, 4 mg/kg/d, administered in two daily IP injections, 8 hours apart. Two hours after the fifth dose (on the third morning), platelets were harvested from these animals and from additional nontreated control animals. The two pools of platelets were then fluorescently labeled with CMFDA and approximately 2.5 × 10⁸ platelets were injected into the tail veins of two groups of normal recipient mice (n = 9 for the control group and 10 for the M-CSF group). The percent of the maximum number of circulating labeled platelets was serially determined for 93 hours by flow cytometry. The mean ± 1 SE is shown for each time point. The mean T_1/2 for control platelet survival was 35.3 ± 0.9 hours ( ± 1 SE) and for M-CSF–exposed platelet survival was 45.0 ± 1.4 hours ( ± 1 SE) (P < .0001). The asterisks indicate a significant difference in the percent of circulating labeled platelets at each indicated time point (P < .05).