Studies of morphologic changes of leukocytes responding to a chemotaxin gradient have been limited due, in part, to difficulties encountered with detailed electron microscopic analysis of cells migrating through membrane filters or on glass slides. The recently developed migration under agarose method provides a new opportunity to view and compare morphologies of cells migrating spontaneously and in response to a chemotaxin gradient. The present report describes the morphologic features we have observed by scanning electron microscopy of human polymorphonuclear leukocytes both spreading on a cover slip and migrating under agarose. We demonstrate that cells migrating under agarose exhibit a morphology that differs distinctly from that of cells spreading on a cover slip and, further, that the cytoplasmic periphery of cells responding to a chemotactic gradient under agarose becomes very irregular due to the formation of blebs or small pseudopodium-like structures.