Flow cytometric analysis. (A) A flow cytometric analysis of whole blood was performed after labeling with a PE-tagged rat antimouse GP Ibα monoclonal antibody. Surface expression of a GP Ib-IX complex requires concomitant expression of GP Ibα, GP Ibβ, and GP IX. As shown, neither blood from GP Ibβ^Null nor GP Ibα^Null animals contains a reactive anti-GP Ibα signal. (B) Mouse integrin, αIIb/β3 levels are shown in mice deficient in a GP Ib-IX complex. A PE-tagged rat antimouse αIIb/β3 monoclonal antibody was mixed with whole blood and samples were analyzed by flow cytometry. An increase in αIIb/β3 levels in heterozygotes and homozygotes probably reflects an increase in platelet volume per platelet. (C) Forward-scatter profile of platelets from the designated mice are presented. Both GP Ibα^Null and GP Ibβ^Null platelets have a similar increased platelet size in their population. GP Ibβ^Het platelet size presents as an intermediate phenotype between wild-type and knockout platelets.