Supernatants of murine lymphoid cultures stimulated with Concanavalin A contain factor(s) capable of sustaining continuous growth of murine lymphoid cells in vitro. Methods for the optimal generation of these growth factors (GM) have been defined, including number of cells required, optimal concentration of Concanavalin A, incubation time, and strain differences in GM production. With GM, cells have been grown for over 15 10-fold generations over a 2-month period. The growing cells express the θ surface marker and do not express surface Ia antigens.

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