GENERAL INTRODUCTION 1954 0.005 Gross beta is a useful criterion of the amount of persisting radioactivity. Plotting the logarithm of this value against the logarithm of the time after detonation depicts the trends of both physical decay and of decline in radioactivity due to a combination of physical decay and various biological and environmental factors. The term decline as used here indicates the trend with time in the amountof radioactivity in successive samples of a particular type of substance at a particular locality. with time. The term is appropriate because the amount usually decreases However, the amount may increase or remain constant for a while if there is an influx of radioactivity into the environment, so that minor fluctuations in decline are to be expected. In this respect de- cline differs from physical decay, which can not remain constant or increase but can only decrease. The three sections of this paper deal respectively with decay data of early samples from Eniwetok Atoll, and with decline of plankton fish samples from Rongelap Atoll. and All have the feature in common that their radioactivity was derived primarily from a single, though not the Same, detonation rather than from two or more successive detonations. -V- ia