Platelet life spans in dogs were measured pre- and post- splenectomy utilizing in vitro whole blood biotinylation. Four splenectomized dogs were found to have significantly lengthened platelet life spans, 193 +/- 7 hours (mean +/- 1 SD;n=4) postsurgery vs. control life spans of 131 +/- 15 hours (n=6; p< 0.01) when analyzed with the multiple-hit model. Additionally, platelets from normal dogs transfused into splenectomized dogs were found to have convex survival curves with extended life spans approximating that of the splenectomized dog. These data indicate that the spleen is a significant determinant of platelet life span in dogs, with survivals increasing approximately 47% upon splenectomy.