Enormous genetic variability is accumulated by plant cells proliferating in culture. Additional variability can be induced in cultured cell populations by exposure to mutagens. This pool of genetic diversity can be examined for agronomically desirable traits at two levels of differentiation. Populations of plants regenerated from callus cultures can be screened by conventional methods. Alternatively, selective culture conditions favoring growth of specific mutant types can be applied at the cellular level. The several characteristics that have been introduced by these methods to date are a harbinger of future contributions to be made by cell culture to the genetic improvement of crops.