P-selectin is an alpha granule membrane associated glycoprotein in platelets (P1) expressed on the surface following exposure to secretagogues in suspension. It is not clear, whether P-selectin is transported from granule membranes to the P1 surface, or released by surface-activation (SfA). In the present study washed P1 were allowed to interact with grids for different periods of time (5-20 min), fixed briefly and exposed to a monoclonal antibody to P-selectin. Grids were washed and exposed to goat anti-mouse IgG antibody coupled to 10 nm gold particles. Examination in the electron microscope revealed a differential distribution of the gold probe on SfA P1. Discoid P1 did not express P-selectin. Early dendritic P1 revealed a few gold probes for P-selectin near the central zone. Late dendritic P1 expressed P-selectin on P1 bodies and some on pseudopods. On fully spread P1 P-selectin probes were evenly distributed, but more concentrated on the peripheral margin than the central zone. Results demonstrate that P-selectin is released from SfA P1. Its initial expression in the central zone suggests it reaches the surface through channels of the open canalicular system. The centrifugal movement of P-selectin is opposite in direction to translocation of mobile receptor-ligand complexes.