1. The protozoon Bodo caudatus grown in counted cultures, which are transferred daily and supplied with a bacterial suspension (grown separately) of known opacity as food, shows a diminution in growth when grown in the presence of radium (400 mg. divided over 38.5 sq. cm. screened with platinum 0.1 mm. and 1.0 mm. of silver).
2. The ratio of the average daily number of generations in a 6-day period in the normal and the continuously irradiated strain is as 100 is to 83.79. The ratio of the average total numbers of bodos on the plates at the end of the daily growth-period of 21½ hours is as 100 is to 65.65.
3. There is no evidence of acclimatization to the effect of irradiation during the periods studied, namely 32 days. A test made with the irradiated strain after 14 days' growth away from radium showed a sensitiveness equal to the normal strain which had never been irradiated.
4. After irradiation for a prolonged period (32 days) with an effective dose of radium, the growth capacity of the irradiated strain is slightly impaired. This difference in growth capacity was observed for 18 days, but it had disappeared 3 months after removal from the radium.
5. The discrepancy in growth of twin plates from the same inoculum, one of which is irradiated, amounts to a ratio for the total figures as 100 is to 57.82 for the average of four pairs at the end of 21½ hours' growth. Expressed in generations the ratio is as 100 is to 81.0.
6. The development of this discrepancy during the daily growth-period is traced, and it is shown that while the irradiation appears to be particularly effective during the first 8 hours of growth, the radium is nevertheless effective at all periods during the 211 hours of growth.
7. The effect of the irradiation is to delay the entry of the irradiated plate into the continuous rapid phase of multiplication by about 2½ to 3 hours in comparison with the twin control.
8. The normal bodos are found to be larger in size in the early hours of a culture than in the later period. This change is traced by giving the mean length and the coefficient of variation in one normal plate at 6 different periods during 23 hours' growth.
9. The bodos at the end of 21½ hours' growth are found to be larger in size in the irradiated plates than in the normal controls.
10. These data are discussed, and the mean lengths and the coefficient of variation for the normal, the irradiated, and each first plate after removal from the radium in a 6-day experiment are given.
11. It is shown that the mean length of the irradiated bodos at the end of 21½ hours corresponds with that of the normal bodos at a much earlier stage (5 hours and 7½ hours). The relation of these data with that derived from the counts of the numbers of bodos in the normal and the irradiated plates is discussed.