Abstract Rabbit muscle enolase stimulates immunoglobulin production by a human hybridoma line, HB4C5 cells under serum-free condition. IgM productivity of HB4C5 cells was enhanced more than 20-fold by this enzyme at 220 μg/ml. Human peripheral blood lymphocytes were also facilitated their IgM and IgG productivity in the serum-free medium. However, baker's yeast enolase was ineffective to accelerate immunoglobulin production by HB4C5 cells, in spite of the same specific enzymatic activity as rabbit muscle enolase. There were differences in sensitivities against heat treatment and trypsin digestion between IPSF and enzymatic activities of enolase. These results imply that the immunoglobulin production stimulating effect of rabbit muscle enolase is irrelevant to its enzymatic function and reaction products. This fact also means that this enzyme has another function other than enzymatic one in glycolysis. Rabbit muscle enolase enhanced IgM production of transcription-suppressed HB4C5 cells treated with actinomycin D. Cycloheximide treatment of HB4C5 cells was useless to inhibit the expression of immunoglobulin production stimulating activity. However, inhibition of post-transcriptional process by monensin invalidated the activity of enolase. These findings suggest that enolase from rabbit muscle accelerates the steps between translation and post-translational processes to enhance immunoglobulin productivity. In addition, laser confocal microscopic analysis revealed that enolase from rabbit muscle was subsequently incorporated by HB4C5 cells.