A proteoglycan heparin has been identified and characterized in mouse mast cells grown in vitro in lymphocyte-dependent culture systems. A histamine-containing cell cloned from mouse bone marrow has been identified as an atypical mast cell which contains a unique proteoglycan chondroitin 4-sulfate. The function of heparin may in part be to form a protease-resistant heparin granule which persists in tissue after release from mast cells until phagocytosed, especially by connective tissue fibroblasts. Following phagocytosis of these heparin granules, fibroblasts respond with increased enzyme secretion. Typical and atypical gastrointestinal mast cells have been enumerated throughout the rat gastrointestinal system and correlated with tissue histamine. A protocol to study food allergy has been developed. Patients participating in the mastocytosis protocol reveal a high incidence of Type IV hyperlipoproteinemia, bone involvement, and cardiac disease.