We found a unique thymocyte growth-promoting activity in supernatants (SN) from subclones of the B cell lymphoma CH12.LX. We have tentatively named this activity B-TCGF (for B cell-derived T cell growth factor) and characterized the activity produced by the CH12.LX.4866 subclone. This SN did not induce thymocyte proliferation alone, however, it enhanced both adult and fetal (Day 15 of gestation) murine thymocyte proliferation in the presence of IL-2, IL-4, or IL-7. Other known cytokines were screened for a B-TCGF-like activity using both adult and fetal thymocytes. IL-6 was found to be active only on adult thymocytes, while TNF alpha and GM-CSF were found to be active only on fetal thymocytes. However, neutralizing antibodies against these cytokines did not block the B-TCGF activity present in CH12.LX.4866 using either adult or fetal thymocytes. These observations suggest that the B-TCGF activity is mediated by a novel factor(s). The apparent molecular weight of this novel molecule(s) was 27-50 kDa determined by sizing HPLC.