Mast cells in skin biopsies obtained 1 hour 40 minutes (A) or 2 hours (B) after the SC injection of 25 or 5 μg/kg rhSCF, respectively, in patients no. 3 (A) and no. 1 (B), who had received daily SC injections of rhSCF for 13 days. Reactivity for histamine is indicated by DAO-gold labeling in the cytoplasmic granules of a mast cell that exhibits no morphologic evidence of secretory activity (A); DAO-gold label is absent when the grid containing a section of the specimen was digested with DAO before staining with DAO-gold (B). Original magnifications: (A) ×67,500; and (B) ×75,000.