Twelve volunteer plasma donors were studied so as to determine the extent and duration of in vivo platelet activation caused by automated plasmapheresis. Samples obtained immediately before and after donation were mixed with murine monoclonal antibodies PAC-1 and S12, which bind specifically to activated platelets. Antibody binding on platelets was quantitated by flow cytometry. The change in the mean fluorescence intensity (MFI) (MFI after donation minus MFI before donation) was 61 +/- 23 (confidence interval [CI], 48-75) for PAC-1 and 56 +/- 64 (CI, 19-92) for S12 in plasmapheresis donors, as compared to 0.3 +/- 0.8 (PAC-1; CI, -0.2-0.8) and 0.3 +/- 0.9 (S12: CI, -0.3-0.9) in whole blood donors (p less than 0.05). Additional studies showed circulating activated platelets up to 48 hours after plasmapheresis. In contrast to other data, significant platelet activation was demonstrated following plasmapheresis on an automated machine. None of the donors had clinical complications. Nevertheless, it may be appropriate to delay subsequent plasmapheresis and platelet procurement from such donors until evidence of platelet activation has disappeared.