It usually appeared in two phases separated by an interval of several days. The primary phase occurred 24 to 48 hours after irradiation in almost all (92 of 97) rabbits exposed to 500 r or more and in 4 of 16 exposed to 300 r or less. The secondary phase occurred during the 2nd week in many of the rabbits irradiated with 900 r or more and in a few irradiated with 700 r.

Autopsy cultures failed to demonstrate the presence of infection in rabbits which died or were sacrificed during the primary phase. Bacterial infection was demonstrated, however, in almost all (21 of 24) rabbits autopsied and cultured during the secondary phase.

After the disappearance of the primary phase in rabbits exposed to 700 or 900 r, the secondary phase could be elicited by initiating bacterial infection. Within 6 to 24 hours after intravenous inoculation of E. coli, Cx-reactive protein reappeared in the blood and persisted until death or termination of the experiment. Reappearance of the protein also followed the intravenous injection of killed E. coli but it disappeared again 1 to 2 days later.

The results indicate that the primary phase is elicited by radiation injury per se and the secondary phase by bacterial infection.