[Population kinetics of microcolonies causing "shouldered" survival curves at low LET].

The well-known fact that the dose dependence of cell survival, when measured by the macrocolony-forming test, can be fitted with the multitarget expression is re-investigated by considering the delayed loss of cellular reproductivity in a microcolony. As the reproductive capacity of each single cell of the microcolony must be lost in order to prevent the development of a macrocolony, the reproductive death of a microcolony is in mathematical analogy to the multitarget model and provides the biological explanation for its formal applicability. Published data from colony-size distributions and the model of "latent genetical damage" are utilized to understand the exponential dose dependence of single cell survival in a microcolony. Exponential survival curves obtained at high LET are explained by an undelayed loss of reproductivity in the irradiated cell. The restitution of the "shoulder" in delayed-plating and split-dose experiments is interpreted by repair effects.