4 mately 50% of that of the controls was reached. The level was maintained at approximately 75% of the control values from the 7th week to the end of the critical period of observation (10th week). The values for the lower age group were below those of the older age group throughout most of the period of observation. The absolute lymphocyte count (fig. 1) of the older age group had fallen by the third day to a value of approximately 55% of the control value. This value was maintained through- out the study, with no definite evidence of an upward trend: The values for the younger age group also fell od b T TTT TT T TT T tT Tr T T T MAJUAO CONTROL S-LEURKOCY TES RONGEL AP LEUROGYTES, NEUTROPHILES AND LYMPHOCYTES 0 10 — Levkocy tea, Mtutropndes and Lymphocytes LEUMOCTTES AGE> $149) Sr lL. a ot) i a : i * Sp i : . : : r - mt ,f rot 4 t : e aA et = + Sp WAJURD CONTROLS -NEVTROPHE ES 1 rN of 4 _ ‘ -re MALFURO CONTROLS LYMPHOCYTES * > + “HE QTROPHLE3 A une? MEDS (49) ~~ ao er 4 ~ » “OU 4 ¢ ‘ae e i: LYMPHOCYTES 4 AGE >5149), : PENSAR t Fcounts “cSunis *! an: ua 8 POST EXPOSURE DAT 183 Fig. 1.—Mean leukocyte changes in highest exposure group (54 people). before the third day to a value of approximately 25% of the control value, after which there was a significant upward trend, recovery being more rapid in the younger age group. Since the mean lymphocyte counts remained essentially constant throughoutthe study, the fluctuation in total white blood cell count was a reflection of changes in the neutrophil component (fig. 1). The sequelae of depression of hemopoiesisare generally considered. as: the: major source of morbidity and 9012592