Natural hemolysin titer and avidity were followed in intact and in splenectomized rabbits not irradiated or irradiated with one or more doses of 500-r x-rays to the total body. Mean hemolysin titers and avidity in unirradiated groups of intact and splenectomized rabbits remained more or less constant. Hemolysin titer in intact rabbits declined for about 12 days after irradiation. Recovery then ensued and usually led to enhanced titers during the 3rd and 4th weeks. Recovery was delayed but not

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... paired by a second dose of 500 r 7 days after the first one. Reirradiation during the recovery period (16 or 32 days after the first one) prompted, often after a lag of several days, a second titer decline followed by a second recovery. Hemolysin titers after irradiation of splenectomized rabbits followed the same sequence as in intact rabbits, but the decline persisted for several more days, recovery was more sluggish and significantly enhanced titers were not found 39 days after a single irradiation or 23 days after the second of two irradiations. Avidity of hemolysin, as measured by intercellular transfer, was typically low, varied widely both within groups and with time and was independent of titer. The variability and fluctuations suggested that natural hemolysin levels are maintained by a series of small, independent γM responses yielding hemolysin of varying degrees of specificity for sheep ervthrocytes.