Influence of antiplatelet GP MoAbs on proplatelet formation. Megakaryocyte colonies were derived from peripheral blood CD34⁺ cells (2.6 ± 0.8 × 10⁴cells/mL) stimulated with 10 ng/mL TPO and 10 ng/mL SCF and were collected on the 9th day from culture dishes (353.2 ± 43.9 colonies). The collected megakaryocytes were incubated with each antiplatelet GP MoAb (10 μg/mL) in a 96-well tissue culture plate for 24 or 48 hours. The number of cells evaluated in each well was 360.5 ± 148.7, depending on the individual preparation of cells used. The purity of the GP-IIb-positive cells was 98.6% ± 0.6% per well, by indirect immunostaining. Cells and megakaryocytes with proplatelet formation were counted using an inverted microscope. The proplatelet formation activity calculation is described in detail in the Materials and Methods section. The data are the means ± SEM from duplicate incubations in two experiments using cells from three donors. *Significantly different compared with the controls (P < .0002). **Significantly different compared with the controls (P < .0007). ***Significantly different compared with the controls (P < .03). Incubation time: ▩, 24 hours; ░ 48 hours.