From July, 1959 to January, 1963, 139 human fetal, 87 adult, 12 child and 154 hog thyroid glands were counted in a spectrometer well-counter system capable of detecting 1.5 pc I131/g of tissue. Neck muscle counts served as background. Using published MPD values of 0.5 rems (population) and 1.5 rems (individual), hog thyroid radioactivity exceeded individual MPD once in September, 1959, and once in September, 1962; exceeded population MPD three times in November, 1961, and three times in August-September, 1962. Human fetal counts never reached the MPD and only once rose to more than half adult thyroid radioactivity. Fetal: maternal thyroid radioactivity in a pregnant woman dying suddenly was 1.08 pc/g: 0.82 pc/g. High radioactivity concentration in hog thyroid always preceded high radioactivity in human adult and fetal thyroids. It is concluded that hog thyroid gland monitoring can be used in lieu of human adult and fetal thyroid gland monitoring until very high concentrations of I131 are found in hog thyroid.