23 IV. Fresh Milk from Chicago Dairies Samples of 2 to 3 gallons of fresh milk were obtained monthly from several of the larger.Chicago dairies for one year beginning in March 1955, The results in Sunshine Units are presented in Figure 5 which shows the very interesting seasonal variation in the Sr90 level of forage and dairy products. The January-February 1956 Sr?0 level appears to be a good average of the levels observed over the 1955 growing season, That is to be expected, since over the winter months the cows feed on hay gathered at various tims during the previous growing season. Similarly, the March-April 1955 level mst repre- sent the average for 195h. The sharp rise in the Spring of 1955 may he due in part to leaf retention of fallout from the concurrent Nevada tests. able in terms of other factors. However, the rise may be explain- When first pastured in the Spring, the cows change from feed grown an average of eight months earlier to new growth, representing an increase in Sr90 level due to the total additional Sr90 fallout during the eight months period. Furthermore, the spring pasturage for the most part may be unplowed fields. The fall-off during the summer may be due to several factors: a larger proportion of feed from recently plowed fields, greater root depth and thus lower Sr90 level for more mature plants, and lower Sr70 retention in milk as a result of richer calcium diet, To the extent that leaf retention is involved, it should have lower effect in summer consistent with the lower observed fallout during the summer months (see 1955 Chicago Rain Data, Table 2 above). The increase from the March 1955 level to the February 1956 level is somewhat greater than the corresponding increase in the total Sr70 soil level from July 1954 to July 1955 (see Figure ). The greater increase in milk level may be the consequence of higher fallout, and thus higher leaf retention, in 1955