Sealing of Transwell membranes by platelets is impaired by pneumolysin. Platelets seal Transwell membranes with 3-µm pores. This was impaired in the presence of pneumolysin but not the pneumolysin mutants as indicated by the flow through of BSA-FITC to the lower chamber measured by fluorescence intensity (to enable comparison with the experiments without immunoglobulins, the data are shown here, although they are presented in the text at the end of “Results”). *P < .05; **P < .01; ***P < .001.