Coleus shoots were exposed to 100,000 R of γ-radiation and the fine structure of the apical meristems was examined. Meristems were fixed at various postirradiation times. An ultrastructural body was found associated with irradiated tissue, bound by a single membrane, containing dense osmiophilic bodies, and usually associated with radiation-induced vacuoles. The development of these new bodies, and the effects of both dose rate and light during the postirradiation period on their development were examined. Reduction of the dose rate by a factor of two inhibited the formation of these macrobodies through the 24 hour postirradiation period. Meristems kept in the dark during the 24 hour postirradiation period had macrobodies similar in form to the macrobodies from the meristems of the 16 hour postirradiation period which were exposed to light. Superlethal doses were used to achieve these results. Similarities between our results and those achieved with lower lethal doses are discussed.

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