Alcohol dehydrogenase-I (ADH-I; EC 1.1.1.1) derived from horse liver stimulated IgM production by human-human hybridoma, HB4C5 cells and human peripheral blood lymphocytes. The immunoglobulin production stimulating activity of ADH-I was inactivated by chymotrypsin digestion, even though the enzymatic activity was completely preserved. This fact suggests that the immunoglobulin production stimulating effect of ADH-I is irrelevant to its enzymatic function. This also means that the effect is a novel function of this enzyme. ADH-I enhanced IgM production by transcription-suppressed HB4C5 cells treated with actinomycin D. Moreover, the enzyme was effective to accelerate IgM production by translation-suppressed HB4C5 cells treated with sodium fluoride and cycloheximide. Although the secretion of IgM by HB4C5 cells treated with monensin for suppression of the post-transcription process was inhibited, the intracellular IgM content of the cells was obviously increased by the addition of ADH-I. It is supposed from these results that ADH-I accelerates the translation activity to enhance immunoglobulin productivity. In addition, a laser confocal microscopic analysis revealed that ADH-I from horse liver was subsequently incorporated by HB4C5 cells.