Previous studies showed that lung and skin mast cells do not contain eosinophil granule major basic protein (MBP). However, MBP has been localized by immunofluorescence to mast cells from a recently established human mast cell line. Analysis of MBP in human mast cell-1 cell lysates by radioimmunoassay showed immunochemical similarity to eosinophil MBP as judged by comparison of dose-response regression lines. Based on these findings and other new information about mast cell heterogeneity, we tested whether mast cells contain MBP. Mast cells were preserved in Carnoy's fixative and were identified by staining with rhodamine-conjugated avidin or for chloroacetate esterase or aminocaproate esterase activity. MBP was localized by immunofluorescence to mast cells in 6 of 7 nasal polyps, 4 of 4 ileal tissue specimens, and 12 of 14 cutaneous mastocytosis specimens. Furthermore, by immunoelectron microscopy MBP was localized to mast cell granules in cutaneous mastocytosis lesions. In contrast, normal skin mast cells preserved in Carnoy's fixative did not contain MBP. After injection of MBP into normal skin and fixation in Carnoy's fluid, mast cells became MBP-positive within 3 minutes, suggesting that endocytosis of MBP by mast cells had occurred. These results suggest that human mast cells in several tissues may sequester toxic eosinophil proteins by endocytosis.