Heparin has been shown to prevent inositol 1,4,5-trisphosphate (IP3) binding to its receptor and to inhibit IP3-induced calcium mobilization in a variety of cells. Heparin added to whole blood at a concentration of 1 U/ml prevented thrombin-induced secretion of granule contents and irreversible aggregation of platelets. Heparin (2-15 kDa) had no inhibitory effect on IP3-induced calcium mobilization in Fura 2-loaded, saponin (10-15 micrograms/ml)-permeabilized platelets. None of the commercially available heparin preparations can induce inhibition of agonist-induced calcium mobilization in intact platelets because they are not cell permeant. Mild saponin treatment makes the membrane permeable to IP3, but restricts the action of heparins. Recent observations suggesting heparin's affinity to IP3 binding sites will be of clinical interest if effective cell permeant analogs can be developed.