A modified spleen colony method was used to measure the functional ability of the irradiated spleen to support clonogenic proliferation. This was assayed by counting the number of exogenous colonies formed by normal bone marrow cells injected into recipient mice whose spleens had previously been irradiated. A shallow dose-related decrease in the ability of irradiated mouse spleen to support clonal growth was seen at all times after splenic irradiation. These dose-response relationships do not resemble the usual clonogenic survival curves. The reduction in colony number produced by normal bone marrow cells did not decrease below 0.4 even when the recipient spleen had received as much as 3000 rads. Recuperation of the ability of irradiated mouse spleen to support clonal growth was observed at intervals of 6 and 9 mo. This appeared to be a temporary recuperation, since a second reduction was seen at 12 mo. An absence of repair was demonstrated when 2000 rads was fractionated into two doses delivered 6 hr to 7 days apart.

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