Elevation of platelet-derived PF-4 correlates with the presence of microscopic tumors. Platelets and plasma from mice bearing a nonangiogenic subclone of the human liposarcoma (SW872) were analyzed at the indicated times using SELDI-ToF. The relative levels of PF-4 protein in platelets of non–tumor-bearing mice at time 0 (●; ie, before the implantation of the tumors) were compared with platelet-associated PF-4 on day 19 (■), day 30 (▲), and day 120 (▼). At 19 days, without a palpable tumor, the median level of PF-4 in platelets is 1.7-fold higher than baseline without a corresponding increase in plasma level of the protein. The red symbols within the cluster analysis represent the median peak intensity of 5 to 6 mice plus or minus SEM.