Summary. Fibroblasts have been implicated as culture-competent cells for the mast cell lineage in several species. In man, fibroblast monolayers sustain the ultrastructural phenotype and function of isolated human lung mast cells and permit the differentiation and full maturation of human mast cells from their agranular precursors in cord blood cells. We examined whether development and maturation of the mast cell lineage in man can be achieved by a supply of the soluble products present in fibroblast culture supernatants. Suspension cultures of cord blood cells were supplemented with culture supernatants derived from two different murine fibroblast lines; controls were not supplemented. The cultures were sampled for light and electron microscopy at 6, 7 and 8 weeks. Human mast cells developed in quantity when cultures were supplemented with the supernatants from BALB/c 3T3 fibroblasts, in reduced numbers when supplemented with Swiss Albino 3T3 fibroblast supernatants, and not at all in culture media alone. By ultrastructural criteria, the newly developed mast cells did not achieve full maturity; they did continue to synthesize new granules and to undergo intragranular maturational events. Small numbers of mature basophils persisted in suspension cultures, and many were undergoing piecemeal degranulation. Other cell lineages noted included eosinophils, neutrophils, macrophages and endothelial cells. We conclude that a factor(s) of fibroblast origin permits the differentiation and partial maturation of human mast cells from their agranular precursors in cord blood, but that fibroblasts must be physically present for complete maturation of these lineages to occur.