Btk^-/-/Tec^-/- platelets display reduced spreading responses on a collagen monolayer but exhibit normal responses on fibrinogen. Murine platelets from the 4 genotypes in PRP were allowed to sediment for 30 minutes onto either (A) collagen or (B) fibrinogen-coated coverslips. The sedimentation was allowed to occur as control, in the presence of ADP (3 μM) or with inhibition of Src kinases by PP1 (20 μM). After 30 minutes the sedimented samples were fixed using 3.2% formaldehyde, subsequently washed with PBS, and stained for F-actin using rhodamine-phalloidin. (A) Wild-type (WT), Btk^-/-, and Tec^-/- but not Btk^-/-/Tec^-/- platelets spread along collagen fibers. With the addition of ADP all genotypes showed spreading, and in the presence of PP1 all genotypes displayed inhibition. Platelets sedimenting on collagen were in the presence of 10 μM lotrafiban to remove the effects of GPIIbIIIa. (B) All genotypes show spreading on fibrinogen with or without ADP. Pictures displayed are small fields of view from separate experiments obtained using a × 100 objective and are representative of 3 experiments.