Bacitracin causes reversible inhibition of collagen- but not CRP-XL–induced platelet aggregation. Platelets were preincubated for 10 minutes with the indicated concentration of commercial bacitracin, then agonist was applied at the points shown by arrows: (A) collagen fibers at 0.5 μg/mL and (B) CRP-XL at 0.1 μg/mL. In panel C, platelets were preincubated with or without bacitracin (12 mM or control, respectively) for 1 hour, washed and resuspended in HEPES-Tyrode buffer without bacitracin, and then aggregation was stimulated with collagen (1 μg/mL). Aggregation, measured as described in “Methods,” is shown as an increase in light transmittance, a representative experiment of 3.