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Summary. Antigenic stimulation of spleen mononuclear ceils from contact-sensitised mice produced a soluble factor that enhanced ihe formation of E-rosettes by human lymphocytes in vitro. Production of this murine E-rosette augmenting factor (E-RAF) occurred over a narrow range of antigen concentrations, was hapten-specific, and depended on T-lymphocytes. The factor was not dialysable, in contrast to other substances extracted From normal leucocytes. MuHne E-RAF is thus a lymphokine whose action is not species restricted. It was readily detected 30 min after contact of ceils with antigen. E-RAF formation by spleen cells and delayed-typc hypersensitivity reactions were elicited over a similar period after contact sensitization of mice.

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