New quantitative techniques have been described for rapid plating of isolated single cells from a wide variety of human tissues under conditions such that almost every cell produces a macroscopic colony. These methods make possible new kinds of studies of the growth and genetics of mammalian cells. Application of these methods to problems of growth rate analysis, cell morphology, mutant production and isolation, analysis of the action of ionzing radiations, study of virus-cell interactions, and to certain problems in clinical medicine has been indicated.