1. Mice pretreated with various amounts of heparin exhibit resistance, relative to heparin dose, to the toxic effects of subsequent injections of otherwise lethal doses of 48/80. There is a straight-line relationship between the 50% protective heparin pretreatment doses and the various 48/80 challenge doses. 2. Fibroblasts exposed to the heparin-48/80 complex rapidly sequester and deposit this material as metachromatic granules in their cytoplasm. 3. The possible significance of these results is discussed.