The in vitro addition of alpha-tocopherol to human platelets was evaluated by measuring fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene-labeled platelets and platelet membranes. The fluorescence anisotropy was decreased compared to non-vitamin E-loaded control platelets at temperatures less than 24--27 degrees C but was increased above 27 degrees C. The curve relating temperature to this parameter remained monophasic in alpha-tocopherol-enriched platelets but exhibited a steeper slope, indicative of increased flow activation energy. The latter was linearly related to platelet alpha-tocopherol levels. A possible role of membrane proteins in the mechanism of this change was suggested by the increased anisotropy which liposomes prepared from platelet membrane lipids displayed when they were located with alpha-tocopherol. This modulation of 'apparent membrane microviscosity' by alpha-tocopherol may play a role in the normal effect of this vitamin on human cells.