feted etetenetd -10- of U255 as depicted by Hunter and Ballou (1951). From their Figure 1 the following approximate slopes were obtained graphically: 20-100 days, -1.2; 200-547 days, -1.8, 2-6 years, -1.3; 10-25 years, -0.7; 150-180 years, -3.7. Thus, the mean slope for the Eniwetok samples is slightly steeper than that of Hunter period from 2 to 6 years, and Ballou up to 100 days; then, in the the Eniwetok samples tend to be at first less steep, and toward the end of the period, steeper. Future counting may show whether these curves will level slightly to -0.7 as does Hunter and Ballou's curve at about 10 years. For the first six curves (Table 1) of pre-Mike samples involving products of the Greenhouse tests the curves tend to be steeper than for corresponding periods for post-Mike samples. Contamination of post- Mike samples by the older products from Greenhouse would tend to pro- duce an effect in this direction, but so marked an effect is rather surprising. In contrast to the general tendency of the slopes to become steeper as noted above for the average after 3 years, certain curves a slower decay than average may be of special importance. showing Curve 49 for rat muscle decayed most slowly of all those studied, with a half life during the In gamma last 3 years, of more than 30 years. spectroscopy of this plate only Cs!37 was detected. A listing of those