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Abstract Moloney‐virus‐induced sarcomas, originating in BALB/c and A/Sn mice, were studied with the colony inhibition (CI) assay. Mice inoculated with the virus as adults developed tumors which spontaneously regressed (regressors) while mice inoculated with the same virus dose at an age of 21 days or less died from progressively growing tumors (progressors). Serum from progressors blocked the colony inhibitory effect of lymph‐node cells (LNC) from the regressors. Serum from mice inoculated with the virus as adults had a similar blocking effect as long as the mice carried progressively growing tumors, but it did not block the LNC effect when harvested at the onset of regression or after its completion. When regressor sera were mixed with progressor sera the blocking effect of the latter decreased.
Hellström et al. (Sun,) studied this question.