An in vitro culture method, previously shown to induce the transformation of rat macrophages to mast cells, has been adapted to humans. Adherent peripheral human blood mononuclear cells were cultured in a medium supplemented with 30% L-cell supernatant and 30% horse serum. During the first week of culture, the cells were phagocytic (80%), lacked histamine, were not metachromatic and stained faintly for naphthol-AS-D-chloroacetate esterase, primarily in a paranuclear location. During the first 2 weeks of culture, the cells grew in size, synthesized DNA, became nonadherent and developed cytoplasmic chloroacetate esterase staining which was strongly positive in 50-60% of the cells during the third week. Intracellular histamine levels increased steadily during culture, and electron-dense metachromatic cytoplasmic granules were present during the third week. Human peripheral blood mononuclear cells contain therefore a subpopulation of cells that can differentiate into mast cells-like cells under specific in vitro culture conditions.