The invention relates to methods of selecting a cardiomyocyte from a cell population derived from a whole heart or a differentiated cell population derived from a stem cell without genetic alteration. Specifically, the invention relates to a method of selecting a cardiomyocyte from a cardiomyocyte-containing cell mixture without genetic alteration of a cardiomyocyte, on the basis of a relative content of cellular mitochondria and/or a relative mitochondrial transmembrane potential of the cell. The invention also relates to methods of enriching a cardiomyocyte from a cardiomyocyte-containing cell mixture without genetic alteration of a cardiomyocyte, producing a cardiomyocyte without genetic alteration of a cardiomyocyte, and evaluating the ratio of a cardiomyocyte in a cardiomyocyte-containing cell mixture.
