Human platelets in plasma were subjected to freeze fracture followed by etching. The outer surface of the platelet plasma membrane was exposed by etching but not by fracturing and was clearly identified by use of latex particles as markers. Surface-associated particles, apparently embedded in the plasma membrane, were found to measure from 83 to 332 A in diameter. Similar particles were associated with membranes lining the surface-connected canalicular system and with limiting membranes of storage granules. Fortuitous fractures exposed two inner faces of the plasma membrane, one of which contained greater numbers of surface-associated particles than did the outer surface of the plasma membrane. The second inner face of the plasma membrane contained numerous fibrillar structures measuring up to 770 A in length. Platelets from a congenitally afibrinogenemic patient appeared normal when examined by freeze-etch technics. Normal platelets exposed to a potent antiaggregating agent (REM 10,393) were found to have lost most of the particles associated with the outer surface of the plasma membrane and to have developed numerous defects in this membrane. A possible role for surface associated particles in platelet aggregation was further suggested by the finding that the nonaggregable platelets of two congenitally thrombasthenic sisters were nearly devoid of these structures.