The adherence to collagen of rabbit platelets labeled in vivo with (35)SO(4) (=) has been studied both in vitro and in vivo. The young platelets are labeled with (35)SO(4) (=) 2-3 days after administration of the isotope to the animals. We exposed platelet-rich plasma (ethylenediamine-tetraacetate, EDTA, as anticoagulant), prepared from blood taken from rabbits 54 hr after giving the (35)SO(4) (=), to collagen in vitro. There was a fall in the specific radioactivity of the nonadherent platelets which indicated a selective adhesion of young platelets to the collagen. In experiments designed to have most of the (35)S label in the oldest platelets it was found that exposure of plasma containing these platelets to collagen resulted in an increase in the specific radioactivity of the nonadherent platelets. Similar observations were obtained when glycine-(14)C was used as a platelet label. However, when DF(32)P (di-isopropyl phosphorofluoridate-(32)P), which is thought to label platelets of all ages equally, was used, the adherence of platelets to collagen did not result in any changes in the specific activity of the nonadherent platelets. In in vivo studies in which we infused a collagen suspension into rabbits 54 hr after giving (35)SO(4) (=) we found that the specific radioactivity of the platelets remaining in the circulation fell. This did not occur when we infused the collagen 96 hr after giving the (35)SO(4) (=). The results from these studies indicate that young platelets adhere to collagen more readily than older platelets.