It is known that cells irradiated at increasing ages in the cell cycle yield survival curves with different radiation response parameters, namely D0 and n. The parameters D0 and n have been determined during the first cell cycle of Oedogonium cardiacum and both factors vary in the same direction with cell age, changing by maximum factors of 7 and 25 respectively. Previously it was reported that synchronized cultures of Oedogonium cardiacum cells irradiated late in the first cell cycle yield dose response curves exhibiting a pronounced inflexion point. In this paper, calculated survival data are presented to show how the presence of this inflexion point is influenced by relative changes in both magnitude and direction of the response parameters of two sub-populations (S and G2 cells) whose radiosensitivity is represented by the initial and latter portions of the complex survival curve. An inflexion point is evident in the survival curve if the difference in either of the parameters is relatively large, but furthermore, the inflexion point is much more pronounced if both parameters vary in the same direction with cell age. Conversely, if the parameters vary in the opposite direction, the inflexion point present in the survival curve is minimized.

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