Three kinetically different reactions (R×1, R×2, and R×3) have been distinguished in the photoinactivation of phage T3 in the presence of the dye proflavine. The response of these reactions to the presence of the radical trap l-cysteine has been examined. At dye concentrations equal to or less than 2.2 μg/ml, R×1 was composed of at least two parallel first-order reactions, one cysteine-insensitive (R×1A) and one cysteine-inhibited (R×1B). R×2 was completely cysteine-insensitive (R×2A). The cysteine sensitivity of these reactions changed abruptly at dye concentrations above 2.2 μg/ml. R×1A and R×1B now operated in tandem, rather than simultaneously, with R×1B being confined to the first 1 min at most. R×2, on the other hand, was completely cysteine-inhibited (R×2B). R×3 was inhibited roughly 75 to 80% by saturating concentrations of cysteine regardless of the time of addition of cysteine. The dark inactivation associated with R×3 was inhibited roughly 85% whether the radical trap was added during the light or dark regimes. Changes of initial phage titer did not alter the cysteine sensitivity of a reaction.