The platelet is well known for its importance in the process of blood coagulation and hemostasis. However, only in recent years has it become apparent that this cell also plays an important role in the process of inflammation and in the stimulation of cell proliferation. It is in this latter context that the platelet may play a vital role in the process of atherogenesis (Ross and Glomset 1976; Ross and Harker 1976). The discovery that platelets are important in cell proliferation was made as a result of the observation that serum derived from cell-free plasma lacked the capacity to induce DNA synthesis in cells in culture (Ross et al. 1974). At the same time, however, such plasma-derived serum retains the capacity to sustain many diploid cells in culture in a quiescent and well-nourished state for periods as long as six to eight weeks. Re-addition of whole blood serum to quiescent cultures of cells such as 3T3 or smooth muscle in plasma-derived serum initiates proliferation in these cell populations (Ross et al. 1978) (Figure 1).