Due to the heterogeneous nature of animal organs, primary cell cultures potentially contain more than one type of cell. Interpretation of data arising from studies using these mixed cultures can lead to difficulties, because it may not be possible to ascertain which of the cell types present may have responded to a given treatment. While a number of procedures have been used to enrich the cell population of interest, many scientists have resorted to cloning of cells in order to insure the purity of cell cultures. Three major strategies have been used to produce clones, namely, the dilution technique, cloning ring technique, and robotic cell transfer. Successful cloning is dependent on optimization of attachment substrata, basal media composition, serum source, and growth factor/hormone additions to support proliferation of the cell type at clonal (low) density.