The present study examines biochemical and functional characteristics of platelet density subpopulations together with their ability to mobilise intracellular fibrinogen when activated. Platelets from three healthy volunteers were investigated. The total platelet population was separated according to density in a linear Percoll gradient in a plasma-free milieu containing EDTA that binds soluble Ca(2+). Subsequently, platelets from each individual were divided according to density into 11 or 12 aliquots. In all fractions, we determined platelet count, intracellular P-selectin and the ADP-evoked platelet fibrinogen binding as a measure of platelet reactivity together with the platelet dense body content. The work demonstrates that platelets use stored intracellular fibrinogen when activated. It also shows that the platelet-fibrinogen binding can be initiated in a surrounding depleted of Ca(2+) and fibrinogen. Moreover, the study demonstrates subpopulations of light platelets having increased reactivity and more alpha-granules but less dense bodies. The biological significance of the findings needs to be elucidated.