(A–D) Mice were either Prkca^–/– or littermate-matched wild-type controls. Platelets were labeled in vivo with Alexa Fluor 488, as described in Methods. (A) Platelets (green) composited with bright field images (black/white) of the cremaster arteriole were viewed, and images were acquired using a digital CCD camera (SensiCam II; Cooke Corp.) with a 640 × 480 pixel array. Original magnification, ×40. (B) Traces shown are median integrated platelet fluorescence of 15 thrombi induced in 3 or more mice for each group. Fluorescent intensity of platelets in arbitrary units is presented as a function of time. (C) Data are presented as a scatter diagram. Horizontal bar represents mean of 15 thrombi induced in at least 3 mice for each group. There is a significant reduction in thrombus intensity in Prkca^–/– in comparison with WT controls (mean Prkca^+/+, 652200; mean Prkca^–/–, 279600; P < 0.05). (D) Time to reach peak thrombus size also significantly differed from WT controls, with mean data shown by the horizontal bars (mean Prkca^+/+, 68.1 seconds; mean Prkca^–/–, 115.2 seconds; P < 0.01). (E) Mice were anesthetized and a transverse incision made with a scalpel at a position where the diameter of the tail was 2.25 to 2.5 mm. The tail was immersed in normal saline (37°C) in a hand-held test tube. The time from incision to cessation of bleeding was recorded, and mean times are shown as horizontal bars. No significant difference was seen comparing WT mice (mean 140 seconds) with Prkca^–/– (mean 117 seconds; P > 0.05).