At fixed pH and ionic environment platelet net charge depends on the composition of surface glycoproteins and phospholipids. It is believed that this composition changes, becoming more negative in the course of activation, and that certain thrombotic disorders such as transient ischemic attacks (TIA) may be associated with platelet activation. To investigate this hypothesis we measured the electrophoretic mobility (U) of normal platelets, activated normal platelets and platelets from patients with TIA and immune thrombocytopenic purpura (ITP). Normal platelets gave U = -0.91 ± 0.05 pm cmh, while normal platelets activated by ADP gave -1.25 and by collagen -1.70. TIA patients (n = 22) had mean U = -1.14 ± 0.10 and the ITP patients (n = 37) had U= -1.07 ± 0.13. The U of both patient groups differed significantly from normal controls, <i>P</i> < 0.001. Indeed, only 2 of the 22 TIA cases had U within 2 SD of the control value. The ITP group included 10 patients with TIA-Like symptoms; the mean U of this subgroup was identical to that of the TIA group without ITP. We conclude that U is a useful measure for research on platelet abnormalities and may even be useful as a routine clinical tool. This work was greatly facilitated by the use of an automated instrument, the DELSA 440, allowing determination of U of a given sample in as little as 5 min.