A case of left atrial myxoma is reported. Some newly recognized clinical characteristics of this tumor are emphasized, and hemodynamic and surgical findings are discussed. The hypergammaglobulinemia in this case was shown to be due to a polyclonal elevation of IgA. Tissue culture of the tumor ceils was successful and revealed the presence of fusion giant cells in the cell culture. Electron microscopy suggested that the endothelial or subendocardial reserve cell was the cell of origin. © 1971.