The mechanism of the macrophage-regulated proliferation of the murine lymphoma cell line FIO 30 has been further investigated. It appears that the macrophage is alone in its ability to support FIO 30 growth; a macrophage-like cell line, however, is unable to provide the stimulus required by the FIO 30 cells for their proliferation. Investigations into the nature of this stimulus indicate that the serum factor pro-MaSF and a macrophage cell surface component act synergistically to support the growth of the FIO 30 cell, but only when the two cell types are in close promixity. MaSF has also been further characterized and is shown to be closely associated with serum albumin.

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