-10- of U235 ag 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 and Ballou up to 100 days; then, period from 2 to 6 years, 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 TaN corresponding periods for post-Mike samples. Contamination of post- Mike samples by the older products from Greenhouse would tend to produce an effect in this direction, but so marked an effect is rather sur- prising. In contrast to the general tendency as noted above for the average of the slopes to become steeper after 3 years, certain curves showing a slower decay than average may be of special importance. 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