The cellular branch of the immune defence in the newborn has been shown to differ from adults in a number of ways. This report presents new data on the functions of the histamine-secreting cells of the newborn. Mast cells of the newborn were obtained from the human umbilical cord by enzymatic dispersion. The granules of the mast cells of the umbilical cord were found to contain both chymase and tryptase by immunohistochemical staining, and the presence of cell-bound IgE on the mast cell surface was demonstrated by staining sections of umbilical cord with peroxidase-conjugated anti-IgE. The enzymatic dispersion yielded 12,660 mast cells per gram umbilical cord (median), range 2,500-60,300 (n = 48). The mast cells were found to constitute 3.1% of the total nucleated cells in the dispersate (median), range 1.5-3.8%. The histamine release from these cells was measured using a glass microfibre-based method. Both the umbilical cord mast cells and the cord blood basophils released histamine stimulated with anti-IgE, concanavalin A and the calcium ionophore A23187. In contrast to mast cells from adult tissue, the phorbol ester TPA was found to be an efficient secretagogue in both mast cells and basophils from the newborn. After maximal stimulation with anti-IgE and phorbol ester the quantity of histamine released per millilitre of blood was significantly higher in cord blood than in adult blood. The spontaneous histamine release from cord blood basophils was also significantly higher than from adult blood basophils. The mast cells found in the umbilical cord matrix and the cord blood basophils represent a readily available source of metabolically active histamine releasing cells for exploration of the role of histamine-secreting cells in newborn immune defence.