Reaction of human smooth muscle antibody with human platelets.

Platelets, prepared from fresh human platelet-rich plasma smeared on slides and stained with human serum containing smooth muscle antibodies (SMA) in indirect IFL, showed a bright cytoplasmic fluorescence with numerous projections extending from the surface. A prerequisite for obtaining a positive reaction with SMA-positive serum was that a chelating agent was present in the suspending medium when preparing the smears. The projections could be demonstrated also by anti-HeLa cell (anti-species) serum. This indicates that the projections had a membraneous cover. Staining of live platelets was always negative. Platelets treated with cytochalasin B for 1 hr were smooth and spherical and did not show any surface projections.