The anticoagulant, EDTA, is known to produce changes in the external contour of platelets. In the present investigation the fine structural alterations which develop in EDTA platelets were examined in the electron microscope. Platelets exposed to EDTA at 37° C. lose their disc shape and become irregularly swollen with multiple pseudopods. This external transformation is identical to the change which occurs in platelets exposed to ADP, thrombin, or low temperature. Internal changes in EDTA platelets, however, are markedly different from those which occur under the influence of aggregating agents or cold. The differences indicate that the effects of EDTA on platelet shape are not due to dispersal of marginal band microtubules or to the trigging of an internal wave of contraction. Chelation of membrane calcium by EDTA appears to cause marked irregularities in the platelet wall and massive swelling of the canalicular system. Swelling of internal channals is the dominant alteration in EDTA platelets, and this change appears ultimately to be related to the breakdown of platelet granules.