ceacam1^−/− platelets demonstrate increased adhesion on immobilized type I fibrillar collagen. (A) Wild-type, ceacam1^+/−, and ceacam1^−/− platelets were allowed to adhere to type I fibrillar collagen (50 μg/mL) in the absence of magnesium, or to plates coated with bovine serum albumin (50 μg/mL) for 15, 30, 45, or 60 minutes at 37°C. After removal of nonadherent platelets, adherent platelets were measured as described in “Methods.” Each assay was performed in triplicate and is representative of 3 independent experiments. Each data point represents platelet adhesion per high-powered field and expressed as the mean plus or minus SEM. Note that wild-type, ceacam1^+/−, and ceacam1^−/− platelets bound to a similar extent on BSA-coated platelets, where at all time points, ceacam1^−/− platelets showed higher levels of adhesion to type I fibrillar collagen than did wild-type and ceacam1^+/− platelets (***P < .005; n = 3). (B) Platelet adhesion on fibrinogen (50 μg/mL) was assessed for wild-type, ceacam1^+/−, and ceacam1^−/− platelets at 15, 30, 45, and 60 minutes at 37°C, as described in panel A.