Binding kinetics of activated GPIIb-IIIa receptors on cultured MKs. Cultured cells were incubated with PE-GPIb antibody and FITC-PAC1 or FITC-FGN in the presence and absence of TRAP or ADP. MFI was determined from MK fluorescence histograms. The percentage of PAC1-positive MKs is shown in parentheses. Fluorochrome equivalents were determined from the MFI as described in Materials and Methods. (A) Effect of agonist concentration on PAC1 binding. Data are representative of 2 experiments performed. (B) Fibrinogen binding kinetics. Arrows indicate fibrinogen concentration at which binding was half maximal (Kd). Data are the mean ± SEM of 3 experiments.