Tables specifically tailored to the exclusion of cytogenetic mosaicism at three confidence levels are presented. The consequences of the assumption of independence in application of the binomial theorem to this question are discussed. The tables are most applicable to the number of cells evaluated from cultures in which all mitoses are arrested in the first in vitro division. For long-term cultures the tables are conservatively applicable to the number of separate colonies evaluated. If n cells have been evaluated from phytohemagglutinin stimulated peripheral blood after 72 hr in cultures, the tables are applicable to between n/2 and n cells.