Summary: At 37°C EAC′1,4,2 becomes unreactive with C′2 according to first order kinetics with a half-life of 10.3 minutes. C′2 is taken up by EAC′1,4,2, as determined by activity measurements and by analyses with radio-iodinated C′3. Under the usual experimental conditions only a small proportion of C′3 is fixed by EAC′1,4,2, leaving a major part free in the fluid phase. From these findings it follows that the reaction between EAC′1,4,2 and C′3 comes to a standstill because of the decay of the cell substrate. The same is true in the reaction between Ea and C′. There is no lag in the reaction between EAC′1,4,2 and C′3, indicating that the resulting formation of a damaged cell is essentially a single event with respect to time. The velocity of reaction between EAC′1,4,2 and C′3 is directly proportional to the concentration of EAC′1,4,2 and varies as the second or third power of the concentration of C′3. This may mean that two or three “molecules” of C′3 must combine simultaneously, or nearly simultaneously, with EAC′1,4,2 for a productive reaction.