An apparatus is described which permitted a perfusant (lactated Ringer's solution) to be passed through a porous sample in a pulsatile manner with a square wave pressure profile. The "on" time, "off" time, number of cycles and pressure amplitude were separately controllable. Using this apparatus and immersing the sample in stirred, heparinized, human blood, there was a certain "off" time below which platelet adhesion to the sample abruptly ceased. The values of this "off" time, termed the activation time ta for platelet adhesion were approximately 0.5 sec for 0.2 micrometers pore size cellulose diacetate/nitrate (millipore filter) and approximately 0.3 sec for 0.2 micrometers polycarbonated (nuclepore filter). After a single cycle with a 5 sec "off" time, adhered platelets on both these materials showed pseudopodia, varying degrees of spreading and membrane perforation.