Studies of tissue factor activity on fibroblasts have found that manifestation of the otherwise cryptic activity is evoked by Triton X-100 or octyl glucoside at concentrations that lyse the cells. Even though sublytic concentrations of the detergents extract membrane lipids into the soluble phase, they were without effect on tissue factor activity. Those experiments led us to conclude that either the fibroblasts maintain plasma membrane lipid asymmetry even as lipids are extracted by the detergents, up to the onset of lysis, or additional mechanisms for regulation of tissue factor specific activity were operative. Using phase contrast and immunofluorescent microscopy, we now show that at least one additional regulatory mechanism is indeed operative. In response to sublytic concentrations of octyl glucoside or Triton X-100, the cells release vesicles from which tissue factor antigen is excluded. Lytic concentrations of the detergents preclude this segregation, leaving only low amounts of tissue factor antigen associated with the adherent cytoskeletons. Two-color staining reveals marked tissue factor-actin filament co-localization, which implies the potential for cytoskeletal participation in the observed tissue factor segregation. We propose that tissue factor activity is indeed regulated by the phospholipids with which it is associated and the degree to which phosphatidylserine is available on the membrane surface, but the cells possess additional mechanisms by which the association of tissue factor with potentially procoagulant membrane domains is controlled.