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A factor in the serum of the marine coelomate, Sipunculus nudus, induced by injecting a mixture of a marine bacterial vibrio and a solution of dried cholera toxin, will, after heating to 85 to 90 C, cause intensive continuous hypersecretion of mucus in isolated free-swimming mucociliated cells from another Sipunculus. The factor, released from coleomic cells into the serum, is heat stable to 90 C, withstands several freeze-thawings, is induced only by specific stimuli, is rapidly released into the serum, persists for different time spans depending on the stimulus, and is not present in normal heated sera. It is proposed that in nature this factor is balanced by a heat labile inhibitory factor. Cholera toxin alone is a feeble stimulus. The marine vibrio alone is a powerful stimulus to mucus secretion but lethal for the host. In combination with cholera toxin, the vibrio is nonlethal.