14 In order to be able to predict the future levels of radioactivity in the plants and animals of the Eniwetok Test Site area, the rate of decline has been determined for many organisms. The rate of decline is defined as the rate at which the radioactivity is decreasing in a given tissue, environment (Held, physical decay. 1957). organ, or organism in its native The only constant factor is that of Prediction of rate of decline would be extremely difficult by other than empirical means. The decline rates and decay rates for many organisms in various areas in the vicinity of the Eniwetok Test Site are reported by Bonham (1958). In general the decay curves are steeper than the decay curve for mixed fission products which is generally expressed as being t7l-2 - The mean of about 100 decay curves was about t “1.5. The greatest deviation from the mean was for muscle samples from birds and rats, for which the decay rate was much slower. The decline rates were equal to or greater than the decay rate of tole? The difference between the decay rate for mixed fission products and the decay rate and decline rate of the biological samples means that the radioactivity is declining faster in plants and animals than it would from fallout that included the entire spectra of fission products. Reasons for this are that there is biological selectivity of the radioisotopes and other factors such as weathering. The decline in radioactivity of representative samples from just after the Nectar shot at Eniwetok