Interferon inhibits the early phase of mitogenic action of Concanavalin A on lymphocytes as measured by incorporation of labelled precursors in RNA, DNA or protein. There was no evidence for a role of cyclic AMP in the process of inhibition. Dialysis of interferon against high salt and urea did not separate cell inhibitory activity from antiviral activity. Thus, these results are compatible with the possibility that the antiviral and cell inhibitory activities are in the same molecule. However, further purification of the interferon will be required to verify this assumption.