[en] Supernatants (sups) from PHA-stimulated sublines derived from the T cell line LBRM-33 were screened for MAF activity (assessed by the activation of thioglycollate-induced PEC's to lyse ¹²⁵I-IUDR labeled FBL target cells). A subline produced sups with high levels of MAF activity but lacking detectable anti-viral activity. γIF, which also exhibits anti-viral activity, is the only well-defined MAF. Therefore, further studies were initiated to determine if LBRM MAF activity was distinct from γIF. Examination of LBRM sups for γIF, in an ELISA capable of detecting 10 units/ml by binding to a monoclonal antibody, detected no γIF. By contrast, sups from Con A-stimulated spleen cells or media containing rγIF which exhibited similar MAF activity to the LBRM sups, contained 315 and 376 units/ml of γIF in the ELISA, respectively. The extent to which γIF potentially contributes to the MAF activity of LBRM sups is being further assessed. The presence of γIF message in LBRM cells is being examined by blot analysis utilizing a probe for murine γIF mRNA. The ability of a monoclonal anti-γIF, which inhibits the in vitro MAF activity of murine γIF, to diminish LBRM-derived MAF activity is being determined. Further studies will attempt to isolate and characterize the lymphokine responsible for the LBRM-derived MAF activity