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Sera from 137 Swiss mice with viral-induced lymphoid leukemia stimulated in vitro granulocytic and mononuclear cell colony formation by DBA/1 bone marrow cells. Only 32% of sera from 180 normal Swiss mice exhibited colony-stimulating activity and this activity was weak compared with leukemic serum. Serum activity was not elevated in the preleukemic period. Linear relationships were found between leukemic or normal serum dose and the number and size of bone marrow colonies developing after stimulation. Filtrates of leukemic tissues exhibited no colony-stimulating activity and inhibited colony stimulation by active sera. Leukemic serum colony-stimulating activity was filterable, withstood repeated freeze-thawing, and was relatively heat stable.
Metcalf et al. (1967) studied this question.