In this study we describe a mast cell population obtained by in vitro culture of hearts of Trypanosoma-cruzi-infected mice. Heart tissue was placed in culture and observed daily for the efflux of cells. Mast cells appeared in the medium during the 1st week of culture in the area immediately surrounding the tissues and increased in numbers over time. The cells were identified as mast cells by electron microscopy and by positive staining of granules with Giemsa, toluidine blue, and berberine sulfate techniques. Histopathological analysis of the cultured heart fragments from infected mice showed numerous mast cells, located mostly in areas where the histologic structure was abnormal and surrounded by fibrous connective tissue. This is the first report on in situ proliferation and migration of mast cells form inflamed heart tissues. In situ grown mast cells might be useful in developing in vitro models to study the role of these cells in T. cruzi-induced and other myocardiopathies.